binman: Mark mkimage entry missing when its subnodes is missing

Using the mkimage entry with the multiple-data-files prop and having a
missing external blob result in an unexpected ValueError exception using
the --allow-missing flag.

  ValueError: Filename 'missing.bin' not found in input path (...)

Fix this by using _pathname that is resolved by ObtainContents for blob
entries, ObtainContents also handles allow missing for external blobs.

Mark mkimage entry as missing and return without running mkimage when
missing entries is reported by CheckMissing.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>