The i.MX Processors support two different boot modes, the internal
boot mode and the external boot mode. Traditionally the external
NAND boot mode is handled in drivers/mtd/nand and the internal
boot mode is handled in arch/arm/mach-imx. This patch consolidates
the handling of both boot modes in arch/arm/mach-imx so that
the user does not have to look in the mtd kconfig section for
booting from NAND. Also, selecting between internal and external
boot mode now is a clear choice.
The external NAND boot mode has been independent of the mtd nand
driver, but as the code was contained in the NAND driver it was
not possible to support booting from NAND without a mtd nand driver.
This is changed with this patch.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>