layerman: switch oe-core to angstrom staging branch

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
bblayers: enable meta-minnow

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Merge remote-tracking branch 'origin/angstrom-v2012.12-yocto1.3'