Tunning FirefoxOS init scripts to better fit on Siyahkernel.

Right now, FirefoxOS has an init.rc that is compatible with Android and Gonk (FirefoxOS low level stuff) cause is used with an official Samsung kernel. As your kernel loads the init.rc after it detects what kind of system we are on, we can have a fitter init.rc script.

The main changes are:
* No Android/Gonk recognition hack is needed on init.rc cause primary/secondrom nature of Siyahkernel (it already does)
* Some services needs to be launched manually (or not to disable them).
* Some other services like installd and zygote are not needed on FirefoxOS
* /data partition is mounted as real device (rw), cause all b2g stuff is written into it's own directory and do not conflict with other Android systems, and having to reflashi Gaia in every reboot is a pain...