Add a new flag --image which can be used to mount the rootfs generated
with "pmbootstrap install".

For now this is quite limited in scope. But it's enough to allow for
building a package, updating it in the QEMU image, and then booting it.

The major "gotcha" with this is that the QEMU uses the kernel and
initramfs from the device chroot unless you run it with --efi.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>