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.

build-all

@@ -4,18 +4,15 @@ set -eux
 # Will take forever from a clean repo, this is most useful
 # after pulling the repository to do an incremental build
 # then quickly test out all the setups.
-gem5=true
+gem5=-g
 while getopts G OPT; do
   case "$OPT" in
     G)
-      gem5=false
+      gem5=
       ;;
   esac
 done
 shift "$(($OPTIND - 1))"
 for arch in x86_64 arm aarch64; do
-  ./build -a "$arch" -klq
-  if "$gem5"; then
-    ./build -a "$arch" -Gkl
-  fi
+  ./build -a "$arch" -klq $gem5
 done