growroot-almost should be inhibitable from boot partition

[Dave Jones]

Some users wish to handle post-boot partitioning or fs configuration themselves. For this use-case it is important that the rootfs is *not* expanded on first boot. On the server images, this is relatively trivial by appending "growpart: off", but on the desktop images it's handled by the growroot-almost service and this is only capable of being inhibited by touching /etc/growroot-grown. It should be possible to inhibit this behaviour from the boot partition only which, being FAT, can be mounted from other OS'.