Tweak the preceeding commit to:
* Add STARTTIME to the identifier to make it unique
* Add MACHINE to the identifier
* Use LOG_DIR
* Store the layer config in a more natural json format
* Drop '_' function prefixes
(From OE-Core rev: 31f0c5e59c7fb0ae0915de584fbfcf3d95bbb061)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>