fs_mgr: Support filesystem labels in fstab

When mounting fstab entries it is practical to be able to specify
for example LABEL=data instead of a specific block device.

This is particularly important for the new Android Emulator code base,
which uses virtio block devices for the various partitions
(system,data,cache), because there is no defined ordering between the
way the filesystems are specified to the emulator and how they are
enumerated by the android kernel as /dev/vdX.

Change-Id: I7aef95f12e8f7b02ac2e33804ba7897fdcb9ad7f
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>