Which bootmode is selected has no longer to be chosen by Kconfig. The
boards can decide themselves which bootmode they want to support. This
makes it unnecesary to ask the user which bootmode shall be supported,
so the "Select boot mode" becomes invisible and both support will be
compiled in as needed by the boards. NAND_IMX_BOOT goes away and the
already existing ARCH_IMX_EXTERNAL_BOOT_NAND can now be used for the
boards to depend on external nand boot.

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