toaster.bbclass: do not fail on non-existent files
Toaster may look up inexistent file paths in the build history
for packages that have been referenced but not built.
This triggers a failure, and this patch recovers by deleting
the reference to the non-built packages.
[YOCTO #6063]
Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com> Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
