buildinfohelper: discover kernel artifacts correctly

buildinfohelper: discover kernel artifacts correctly

Because some image_license.manifest files contain multiple
FILES lines, and because those lines can sometimes not contain
a list of files (i.e. they look like "FILES:\n"), we were
resetting the list of kernel artifacts when we hit the second
"empty" line.

Fix by ignoring any FILES line which doesn't list files, and by
appending any files found in a valid FILES line, rather than
overwriting the existing list.

[YOCTO #10107]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>