luci-app-attendedsysupgrade: request filesystem

It is possible to request a specific filesystem so no other filesystems
are used. This speeds up the build process and may prevent failures in
edge cases.

A recent edge case is installing more packages than ext4 can handle
while squashfs works fine due to compression.

Since  `rootfs_type` is now always available via `procd`, remove the
hack of reading `/proc/mounts`.

Signed-off-by: Paul Spooren <mail@aparcar.org>