superiotool: Allow to override Makefile variables `CC`, `INSTALL` and `PREFIX`

This way for example a different compiler can easily be used.

    CC=clang make

Change-Id: I50b83554fd4826d00d87e60a30eb1f6a88834397
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/2935
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
1 file changed