bitbake: fetch2: avoid cache ignoring missing files

bitbake: fetch2: avoid cache ignoring missing files

Previously, if a file listed in SRC_URI was missing at the time of
parsing, and then was added and bitbake run again, it would not be
picked up because the recipe was cached without it. If we allow the file
to be added to the list of files to checksum, then it will be checked
for and found on the second run.

Fixes [YOCTO #4790].

(Bitbake rev: 71da822762cb298261cccdfa54b9c0fea02c3c5d)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>