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>
---
One other possible way to resolve this would be another BINMAN_something
flag to pass to make to in turn pass --ignore-missing as well to binman
as the use case here is platforms such as allwinner than require BL31
but have other optional firmware (SCP).