binman: Add a Makefile for test-program compilation

These test programs are includedd as binary files in U-Boot to avoid
having to build them (and associated toolchain differences). Instructions
on building are in the files themselves, but it seems better to provide
a Makefile which can be manually run when desired.

Add a Makefile, separate from the normal build system, to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>