classes/base: add checksum file list at parse time
Connect the new fetcher file checksum code so that we get a list of the
files to be checksummed at parse time.
The file-checksums flag will not be read unless we are using a version
of BitBake that supports the function we call within it, so it is safe
to include this change even when the metadata will still be used with
older versions of BitBake.
Implements [YOCTO #2044].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
