This reverts commit 4b17d73c7da2, as it is incomplete and partially
broken. ARCH_IMX_EXTERNAL_BOOT_NAND does not depend on MTD, it just uses
some defines from the mtd/nand header, but does not actually depend on MTD
being compiled in.

For the other two cases there is a more complete fix merged with commit
57b584d748d4 that also enables the needed MTD write support.

Fixes:
(MACH_TX25 && MACH_PCA100 && MACH_PCM038) selects ARCH_IMX_EXTERNAL_BOOT_NAND
which has unmet direct dependencies (ARCH_IMX && MTD)

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>