Makefile: Show binman missing blob message

Makefile: Show binman missing blob message

When binman is invoked during a build of U-Boot and an external blob is
missing, the user is usually presented with a generic file not found in
input path message.

Invoke binman with --allow-missing so that binman can show relevant
missing blob help messages. Build continue to fail with missing blobs
unless BINMAN_ALLOW_MISSING=1 is used, same as before.

This changes the following error message during a normal build:

  binman: Filename 'atf-bl31' not found in input path (...)

to the following:

  Image 'itb' is missing external blobs and is non-functional: atf-blob

  /binman/itb/fit/images/atf/atf-blob (bl31.bin):
     See the documentation for your board. You may need to build ARM Trusted
     Firmware and build with BL31=/path/to/bl31.bin

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>