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.

build-baremetal

@@ -76,7 +76,7 @@ def main(args, extra_args=None):
             gcc = 'arm-none-eabi-gcc'
         else:
             os.environ['PATH'] = common.crosstool_ng_bin_dir + os.environ['PATH']
-            gcc = common.get_toolchain_tool('gcc')
+            gcc = common.get_toolchain_tool('gcc', allowed_toolchains=['crosstool-ng'])
         if args.gem5:
             if common.machine == 'VExpress_GEM5_V1':
                 entry_address = 0x80000000