bitbake.conf: exclude DATETIME var dependency from IMAGE_NAME

bitbake.conf: exclude DATETIME var dependency from IMAGE_NAME

* resolves ERROR shown when bitbake -S is used for image:
  ERROR: Bitbake's cached basehash does not match the one we just generated
  (/OE/oe-core/openembedded-core/meta/recipes-core/images/core-image-minimal.bb.do_rootfs)!
  ERROR: The mismatched hashes were 8c35cdf8a5d09c03941f081dd9f6d8dc and b5d6e2e5952770557c48c5779ddb73fc

(From OE-Core rev: 084d16cd00365ac3db9010a2e0e08ec49b50a144)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>