composite: move "factory" target to custom script
It's mostly the same and the indirection through custom
is annoying to work with. Since it is just a helper for
the VM image type don't document it further.
build: better PRODUCT_ZFS handling
Only VM images really handle ZFS. While here move the handling
to the core repository so rc.loader knows what to do.
We only need a 10 second boot relay on installer vga/serial/dvd/nano.
Also go back to setting the pool name using PRODUCT_ZFS which is
now safeguarded through ALLOWED/REQUESTED.