Add EXTRA_BAREMETAL_OPTS to stack.sh

Allow the passing of extra options to the [baremetal] option group
by specifying EXTRA_BAREMETAL_OPTS, using the same format as the
existing flag EXTRA_OPTS.

Change-Id: I209675786c6a33a68d83a371292a1e1749ecb14c
Add stackrc comments to HACKING

Change-Id: I46ff885184a2b5b71caca905c27f28d8b1304011
Merge "Add EXTRA_BAREMETAL_OPTS to stack.sh"
Merge "Add stackrc comments to HACKING"