src/vendorcode/amd: Use AR variable in Makefiles

Change-Id: I5158f1bcc18eb5b15f310d0cf50fb787c12317c8
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/21700
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
6 files changed