update.py: check whether branch existed when nocheckout

update.py: check whether branch existed when nocheckout

Fixed:
Assume there is no master branch in hello layer:
$ update.py -l hello -b master
INFO: Skipping update of layer hello - branch master doesn't exist

This is correct since hello layer doesn't have master branch, but when --nocheckout:
$ update.py -l hello -b master  --nocheckout
[snip]
INFO: Sorting layers for branch mater:
WARNING: Cannot find required collections on branch master:
WARNING: hello: LAYERDEPENDS: <snip>

This is incorrect, this patch fixed the problem, now it skips it since the
branch doesn't exists when --nocheckout.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>