Makefile: With BINMAN_ALLOW_MISSING=1 don't error on missing

When the user builds with BINMAN_ALLOW_MISSING=1 they're explicitly
setting the flag to allow for additional binaries to be missing and so
have acknowledged the output might not work. In this case we want to
default to not passing a non-zero exit code.

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>