Microcode: Show a useful warning when microcode bins are missing

Because the binary repo is disabled by default, we get frequent
questions about why the build failed, relating to microcode in the
binary repository.

- Show an error saying that the file is missing instead of the typical
make error of no rule to build the file.
- Show a note encouraging users to try enabling the binary repo if it's
not enabled.

Change-Id: If4148c18cfb781ed2932bd2ae4a289b621afdebf
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/17940
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>
1 file changed