FLASH_START is supposed to point at the memory area where NOR flash are
mapped. We currently have an incorrect FLASH_START copied from ar71xx
back then and the loader doesn't work under OKLI mode.
On ramips, mt7621 has it's flash mapped to 0x1fc00000 and other SoCs
uses 0x1c000000. This commit makes FLASH_START a configurable value to
handle both cases.
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>