fs: Implement fstab devicetree integration for EXT2

Device tree and/or automount support was not present and might be useful
when for example the settings module needs early access to a filesystem.
This patch adds support to mount an EXT2 filesystem automatically. The
implementation chosen is to provide the drive name matching a disk
drive specified in the device tree ie on an (e)MMC or SD card.

Signed-off-by: Bas van Loon <bas@arch-embedded.com>