fs/skeleton: allow rootfs not to be remounted read-write

[Peter: prefix option with TARGET_GENERIC_]
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>