layout.conf: add git friendly pregenerated cache format

layout.conf: add git friendly pregenerated cache format

Enabled via cache-format = md5-dict
This format is essentially just flat_hash, using md5 rather than mtime,
and dropping the path component from _eclasses_ entries.

From a speed standpoint, the md5 overhead is ~16% in comparison to mtime,
timed on a modern sandybridge; specifically, validating 29k nodes takes
~8.8s for flat_md5, while the pms norm is ~7.7s.

That said, the cache is /usable/ in places PMS is not; in those cases,
it can definitely be a win since even if the cache is partially old,
it's better than regenerating everything from scratch.
(cherry picked from commit 95ddf97e2f7e7d3f6a072604b2df5f77e9298558)

Change-Id: Ic3561369b7a8be7f86480f339ab1686fddea6dff