meta/lib/oe/recipeutils.py: Handle multi-repo recipes for upgrade check

meta/lib/oe/recipeutils.py: Handle multi-repo recipes for upgrade check

For a recipe that uses more than one git repo there isn't a single
SRCREV variable. For example for linux-yocto there is SRCREV_machine and
SRCREV_meta and rd.getVar("SRCREV") yields "INVALID".

Luckily bb.fetch2 already handles all the details and exposes the
currently used revision in ud. So just use that.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>