gpt-auto-generator: do not apply image policy on the root fs and /usr/ fs

gpt-auto-generator: do not apply image policy on the root fs and /usr/ fs

At the moment the gpt-auto generator does its things we already
transitioned into the host OS, i.e. the root fs and /usr/ are mounted.
Hence suppress image policy checks for those two partitions.

This actually matters, because the root hash/usr hash is taken into
consideration for the image policy checks, but we don't have that in
gpt-auto and hence would refuse operation claiming policy conflicts
event though we never actually operate on the root fs via the dissection
logic.