utils: add common function to check out a specific git revision

Checking out a revision in the bitbake/layer repositories is something
we are doing in a few places, so add a checkout_repo() function that
does this, ensuring that we don't get tripped up by any junk files,
and avoids checking out if the repository is already at the desired
revision (thus avoiding the clean operation and e.g. preserving any
.pyc files that aren't stale and would speed things up a little). Note
that we do the clean before checking out in case there are untracked
files that are tracked in the commit we are checking out.

In addition to adding this function, change the existing code that we
use in the update script to check out a layer use the new function.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

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