Adds support in the zephyr port to mount a file system if a block device
(sdhc disk access or flash area) is available. The mount point is either
"/sd" or "/flash" depending on the type of block device.
Tested with an sdhc disk access block device and fatfs on the
mimxrt1050_evk board.
Tested with a flash area block device and littlefs on the reel_board.
5feb54afbb