Use the HS hook to completely generate the HS BL descriptor, similarly
to what is done in the LS hook, instead of (arbitrarily) using the
acr_v1 format as an intermediate.

This allows us to make the bootloader descriptor structures private to
each implementation, resulting in a cleaner an more consistent design.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>