binman: add faked optional entry case in CheckForProblems

binman: add faked optional entry case in CheckForProblems

When having an entry that is marked as optional and is missing in the
final image, the following output is observed:

  CFGS    spl/u-boot-spl.cfgout
  BINMAN  .binman_stamp
Image 'image' has faked external blobs and is non-functional: tee.bin

Image 'image' is missing optional external blobs but is still functional: tee-os

/binman/section/fit/images/tee/tee-os (tee.bin):
   See the documentation for your board. You may need to build Open Portable
   Trusted Execution Environment (OP-TEE) and build with TEE=/path/to/tee.bin

Some images are invalid
make: *** [Makefile:1135: .binman_stamp] Error 103

To solve this contradictory messaging, when checking the faked blob
list, remove entries that are allowed to be missing. Instead add an
info message for faked optional blobs. Also reduce verbosity of the
optional image warning to an info message.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Bryan Brattlof <bb@ti.com>