update.py: add --nocheckout option
update.py: add --nocheckout option

This allows disabling the checkout of the appropriate branch so that we
can test the update script with fixes to bitbake or the metadata if
needed.

Signed-off-by: Paul Eggleton <737d4dae46a541673d7bf4e0d5b32714b16528d8@linux.intel.com>

bartosh/layerindex-web,bartosh/layerindex-web,bartosh/layerindex-web