block: don't probe mtdblock on NAND (with legacy exceptions)

block: don't probe mtdblock on NAND (with legacy exceptions)

Recent kernels started to spill warnings on the log if a userspace
process open()s an mtdblock device backed by NAND flash:

mtdblock: MTD device 'foo' is NAND, please consider using UBI block devices instead.

The warning itself is legitimate -- one really shouldn't be using
mtdblock on NAND.
Hence make fstools skip probing mtdblock devices if their underlaying
mtd device is of type 'nand'. As we don't want to break boards actually
using JFFS2 and squashfs directly on NAND, still probe the mtdblock
device in case the mtd device name is 'rootfs' or 'rootfs_data'.
This will then also trigger the kernel warning as it should.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>