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.

bisect-linux-boot-gem5

@@ -24,7 +24,7 @@ args = common.setup(parser)
 # We need a clean rebuild because rebuilds at different revisions:
 # - may fail
 # - may not actually rebuild all files, e.g. on header changes
-common.rmrf(common.linux_variant_dir)
+common.rmrf(common.linux_build_dir)
 assert build.main(args) == 0
 status = run.main(args, {
     'eval': 'm5 exit',