build: unify QEMU and gem5 kernel configs! The last one!

Split optional and required kernel configs into separate files.

readme: describe how our kernel configs are generated from Buildroot

readme: document QEMU `-no-reboot` and speculate gem5 `panic_on_panic`
in preparation for some more automated bisection with ktest.

kernel_config_fragment/default

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+CONFIG_BLK_DEV_INITRD=y
+CONFIG_DYNAMIC_DEBUG=y
+CONFIG_MODULE_SRCVERSION_ALL=y
+CONFIG_OVERLAY_FS=y
+
+# GDB debugging.
+CONFIG_DEBUG_FS=y
+CONFIG_DEBUG_INFO=y
+CONFIG_DEBUG_KERNEL=y
+CONFIG_GDB_SCRIPTS=y
+
+# Non-static variables show up on /proc/kallsyms
+# https://stackoverflow.com/questions/20196636/does-kallsyms-have-all-the-symbol-of-kernel-functions/44614878#44614878
+CONFIG_KALLSYMS_ALL=y
+
+# /proc/config.gz
+CONFIG_IKCONFIG=y
+CONFIG_IKCONFIG_PROC=y
+
+# TODO make example.
+# This seems to allow userspace to create arbitrary configuration trees,
+# which kernel modules can then read and interpret.
+CONFIG_CONFIGFS_FS=y
+
+# KGDB
+CONFIG_CONSOLE_POLL=y
+CONFIG_KDB_CONTINUE_CATASTROPHIC=0
+CONFIG_KDB_DEFAULT_ENABLE=0x1
+CONFIG_KDB_KEYBOARD=y
+CONFIG_KGDB=y
+CONFIG_KGDB_KDB=y
+CONFIG_KGDB_LOW_LEVEL_TRAP=y
+CONFIG_KGDB_SERIAL_CONSOLE=y
+CONFIG_KGDB_TESTS=y
+CONFIG_KGDB_TESTS_ON_BOOT=n
+CONFIG_MAGIC_SYSRQ=y
+CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1
+CONFIG_SERIAL_KGDB_NMI=n
+
+# Module.symvers in kernel tree and modules tree contains CRC of signatures.
+# TODO: I think the CRC are stored in the built kernel and module, and checked
+# at insmod, but bgrep did not find it in kernel image.
+# Does not show in /proc/kallyms either.
+CONFIG_MODVERSIONS=y
+
+# ftrace
+CONFIG_DYNAMIC_FTRACE=y
+CONFIG_FTRACE=y
+CONFIG_FTRACE_SYSCALLS=y
+CONFIG_FUNCTION_GRAPH_TRACER=y
+CONFIG_FUNCTION_PROFILER=y
+CONFIG_FUNCTION_TRACER=y
+CONFIG_HWLAT_TRACER=y
+CONFIG_IRQSOFF_TRACER=y
+CONFIG_SCHED_TRACER=y
+CONFIG_STACK_TRACER=y
+CONFIG_TRACER_SNAPSHOT=y
+
+# Process tracing.
+CONFIG_CONNECTOR=y
+CONFIG_PROC_EVENTS=y
+
+# 9P
+CONFIG_9P_FS=y
+CONFIG_9P_FS_POSIX_ACL=y
+CONFIG_9P_FS_SECURITY=y
+CONFIG_NETWORK_FILESYSTEMS=y
+CONFIG_NET_9P=y
+CONFIG_NET_9P_DEBUG=y
+CONFIG_NET_9P_VIRTIO=y
+# 9P needed for ARM. Not fully minimized, but so be it.
+CONFIG_PCI=y
+CONFIG_PCI_HOST_COMMON=y
+CONFIG_PCI_HOST_GENERIC=y
+CONFIG_VIRTIO_PCI=y
+CONFIG_VIRTIO_BLK=y
+CONFIG_VIRTIO_NET=y
+
+# Misc
+#CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y
+
+## Networking
+
+# Will everything blow up?
+# https://superuser.com/questions/684005/how-does-one-permanently-disable-gnu-linux-networking/1255015#1255015
+#CONFIG_NET=n
+
+# If given, we can use QEMU 2.9.0 default x86 networking without any -net or -netdev options,
+# since E1000 is the default networking device as mentioned at:
+# https://en.wikibooks.org/w/index.php?title=QEMU/Networking&oldid=3268753
+#CONFIG_E1000=y
+
+## x86
+
+# https://stackoverflow.com/questions/20069620/print-kernels-page-table-entries
+# cat /sys/kernel/debug/kernel_page_tables
+CONFIG_X86_PTDUMP=y
+
+## UIO
+
+# Userspace drivers: allow you to handle IRQs and do memory IO from userland through a /dev file.
+#
+# Superseded by the more featureful VFIO.
+#
+# Documentation/DocBook/uio-howto.tmpl contains actual userland examples
+# for the generic examples under drivers/uio
+#
+# UIO interface in a nutshell:
+#
+# - blocking read / poll: waits until interrupts
+# - write: call irqcontrol callback. Default: 0 or 1 to enable / disable interrupts.
+# - mmap: access device memory
+
+# All other UIO depend on this module.
+CONFIG_UIO=m
+
+# Generic platform devices.
+# https://yurovsky.github.io/2014/10/10/linux-uio-gpio-interrupt/
+CONFIG_UIO_DMEM_GENIRQ=m
+CONFIG_UIO_PDRV_GENIRQ=m
+
+# https://github.com/rumpkernel/wiki/wiki/Howto:-Accessing-PCI-devices-from-userspace
+# /sys/class/uio/
+# /sys/class/uio/uio0
+CONFIG_UIO_PCI_GENERIC=m
+
+## ARM
+
+# LEDs:
+#
+#     cd /sys/class/leds/versatile:0
+#     cat max_brightness
+#     echo 255 >brightness
+#
+# https://raspberrypi.stackexchange.com/questions/697/how-do-i-control-the-system-leds-using-my-software
+#
+# Relevant QEMU files:
+#
+# - hw/arm/versatilepb.c
+# - hw/misc/arm_sysctl.c
+#
+# Relevant kernel files:
+#
+# - arch/arm/boot/dts/versatile-pb.dts
+# - drivers/leds/led-class.c
+# - drivers/leds/leds-sysctl.c
+#
+# Try hacking QEMU's `hw/misc/arm_sysctl.c` with a printf:
+#
+#     static void arm_sysctl_write(void *opaque, hwaddr offset,
+#                                 uint64_t val, unsigned size)
+#     {
+#         arm_sysctl_state *s = (arm_sysctl_state *)opaque;
+#
+#         switch (offset) {
+#         case 0x08: /* LED */
+#             printf("LED val = %llx\n", (unsigned long long)val);
+#
+# to obeserve when the callback is made. But beware that one of the LEDs
+# has a heartbeat trigger by default (specified on dts), so it will produce a lot of output.
+CONFIG_LEDS_CLASS=y
+CONFIG_LEDS_CLASS_FLASH=y
+CONFIG_LEDS_SYSCON=y
+CONFIG_LEDS_TRIGGERS=y
+CONFIG_LEDS_TRIGGER_BACKLIGHT=y
+CONFIG_LEDS_TRIGGER_CPU=y
+CONFIG_LEDS_TRIGGER_DEFAULT_ON=y
+CONFIG_LEDS_TRIGGER_GPIO=y
+CONFIG_LEDS_TRIGGER_HEARTBEAT=y
+CONFIG_LEDS_TRIGGER_ONESHOT=y
+CONFIG_LEDS_TRIGGER_TIMER=y
+CONFIG_NEW_LEDS=y
+
+# GPIO
+CONFIG_ARM_AMBA=y
+CONFIG_GPIOLIB=y
+CONFIG_GPIO_SYSFS=y
+CONFIG_GPIO_PL061=y
+
+# In target:
+#
+#     modprobe dummy-irq irq=34
+#     insmod /platform_device.ko
+#
+# Outcome:
+#
+#     lkmc_platform_device_write offset=0 value=12345678 size=4
+#     dummy-irq: interrupt occurred on IRQ 34
+#
+# When the device generates an IRQ, the dummy module also snoops it.
+#
+# The IRQ number 34 was found by via dmesg on a previous "insmod /platform_device.ko".
+CONFIG_DUMMY_IRQ=m
+
+# Like CONFIG_X86_PTDUMP for ARM.
+CONFIG_ARM64_PTDUMP=y
+
+# For record and replay.
+CONFIG_8139CP=y