arm: mvebu: Convert BOOT_FROM_* constants to function macros

This allows to merge BOOT_FROM_MMC and BOOT_FROM_MMC_ALT constants to one
macro. And also allows to extend other BOOT_FROM_* macros for other
variants.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Tested-by: Martin Rowe <martin.p.rowe@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>