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.
Backport `procd` detection of `rootfs_type` to JavaScript:
https://git.openwrt.org/?p=project/procd.git;a=blob;f=system.c;h=
93eac59c3b01ce3729dc27539ac483f5314759d3;hb=HEAD#l49
Signed-off-by: Paul Spooren <mail@aparcar.org>