Portability improvement

Makefile.inc uses $( ) syntax on the shell. That's isn't as universal
as one would like.

Change-Id: I9a8fd511eef7fefc1458d5bae2cd7ef5475b7392
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/777
Tested-by: build bot (Jenkins)
Reviewed-by: Bernhard Urban <lewurm@gmail.com>
1 file changed