handle mounts in recovery if the have only one argument

controlled by BOARD_RECOVERY_HANDLES_MOUNT if it is not set
the recovery will behave like before.

this allows the recovery to handle commands like 'mount system'
it is needed for devices with two different filesystem in
recovery.fstab like RFS and EXT4 cause the regular recovery
behaviour will only generate a fstab for mounts of fstype2
and ignore the other fstype. This will also enable things like

run_program("/sbin/busybox", "mount", "/system");

on those system.

Change-Id: Ib10ffc7735a2edb8dd32be230ba885d5d2744f73