imx: ventana: use EEPROM register for falcon boot mode

imx: ventana: use EEPROM register for falcon boot mode

NAND+MMC env support costs 12KB in the SPL which is fairly expensive just
for the ability to specify whether or not to boot to uboot or directly
to linux. The Ventana boards have plenty of EEPROM storage so we will use
a byte there to signify if we should boot to the bootloader or to the OS.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>