binman: Use the Makefile to build ELF test files

At present the ELF test files are checked into the U-Boot tree. This is
covenient since the files never change and can be used on non-x86
platforms. However it is not good practice to check in binaries and in
this case it does not seem essential.

Update the binman test-file Makefile to support having source in a
different directory. Adjust binman to run it to build bss_data, as a
start. We can add other files as needed.

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