Using ud.localpath as a basename when it points at a directory causes
problems. The supports_checksum() method gives a good indication of whether
ud.localpath can be used in the way we need.
(From Poky rev: 933ec8a44634e33f92f6f76de3a34094c3d63aa6)
(Bitbake rev: dcd79ae20ab2c72c3312b2251c2b6dc4cabe988e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>