Pass "rootfs_bak" to the kernel when booting the backup rootfs

It was previously set to "alt", but "rootfs_bak" is a little more
verbose, and that's what min-init expects.

Conflicts:
	src/main.c
Pass "kernel_bak" param to the kernel when booting the backup

This parameter will be used from userspace by the upgrade
application, to know whether or not a backup kernel can be created.
This ensure that there is always a working kernel on the system.

Conflicts:
	src/main.c