[MIPS] Replace disk_preparing stuff with setup_fs mechanism.

The setup_fs mechanism provides the same facility as the
disk_preparing code that we implemented but additionally
reboots the system starting the init sequence from scratch.

I think that JB-Mr2 seems to be dependant on /data early
and I'm having better results by using this commonly used
mechanism.

Signed-off-by: Pete Delaney <piet.delaney@imgtec.com>