Build the Linux kernel independently from Buildroot

This will allow for other types of root filesystems that don't rely on Buildroot
to be added and used in the future.

Propagate --verbose on all build scripts to see full GCC commands.

build-all: allow for neat subsets

also 9p share rootfs_overlay. TODO document.

kernel_config/default

@@ -2,7 +2,7 @@
 CONFIG_BLK_DEV_INITRD=y
 CONFIG_DYNAMIC_DEBUG=y
 CONFIG_MODULE_SRCVERSION_ALL=y
-CONFIG_STRICT_DEVMEM=n
+# CONFIG_STRICT_DEVMEM is not set
 
 # Filesystems.
 CONFIG_DEBUG_FS=y
@@ -24,10 +24,10 @@ 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_KGDB_TESTS_ON_BOOT is not set
 CONFIG_MAGIC_SYSRQ=y
 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1
-CONFIG_SERIAL_KGDB_NMI=n
+# CONFIG_SERIAL_KGDB_NMI is not set
 
 # Non-static variables show up on /proc/kallsyms
 # https://stackoverflow.com/questions/20196636/does-kallsyms-have-all-the-symbol-of-kernel-functions/44614878#44614878
@@ -89,10 +89,6 @@ CONFIG_LOGO=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
@@ -121,7 +117,6 @@ CONFIG_UIO_PCI_GENERIC=m
 
 ## ARM
 
-# Like CONFIG_X86_PTDUMP for ARM.
 CONFIG_ARM64_PTDUMP=y
 
 # For record and replay.