compose: Support 'boot_location' to facilitate GRUB2

Having content in /boot in OSTree was always ugly, because we ended up
mounting over it in the deployment location at boot.

This was even worse in the anaconda rpmostreepayload code, because of
the juggling of the mount point that needed to take place.

Trying to add a GRUB2 backend to OSTree is what finally forced this
change.  Now, we put kernels (in the tree) by default in *both* /boot
and /usr/lib/ostree-boot.

OSTree itself knows to look in both locations.  Anaconda is going to
just hard require trees with the new location though.