Move GNU_HASH warning fixes here from meta-mentor

Internal toolchains generally don't see these, as they build that toolchain
with the default behavior they expect, so it's appropriate to include these
fixes with the external toolchain configuration, for now.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Add initial layer README

Signed-off-by: Christopher Larson <chris_larson@mentor.com>