binman: mark optional missing blobs as absent

Optional blobs should mark themselves as absent to avoid being packed
into an image.
Extend the documentation of this behaviour. Although the documentation
implied this before, the "optional" property had not been explained
properly before.
The behaviour will change as now absent entries are no longer
packed into an image. The image map will also reflect this.
As a result, the CheckForProblems() function will no longer alert on
optional (blob) entries. This is because the missing optional images
were removed before CheckForProblems is called.
Adjust the testExtblobOptional test case to highlight that we are
testing not only an optional image but the image is missing as well. The
behaviour for these is different where the latter will not be packaged
into the image.

Reported-by: Simon Glass <[email protected]>
Signed-off-by: Yannic Moog <[email protected]>
Reviewed-by: Bryan Brattlof <[email protected]>

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