fetch: Add ability to fast-forward existing bitbake / meta layer git repos.

This is a bit conservative in that we only fast forward and do nothing
if the git repo and the LAYERS file disagree on the branch name.
fetch.sh: Don't pushd/popd unnecessarily for metas dir.
fetch.sh: Quiet down 'git status' when detecting changes in repos.
build.sh: time the build and direct output to a log file.
fetch.sh: Replace pushd/popd with 'cd'.

Better than requiring bash though OE likely already does.
fetch.sh: Fix bad string comparison in test.
fetch.sh: Make directory for metas before trying to 'cd' into it.
Merge remote-tracking branch 'origin/master' into measured

Conflicts:
	build.sh
build.sh: Fix image name.

No clue how this got changed.