The Boot ROM write some boot source ID (SD card, eMMC, SPI, ...) into
a certain location in SRAM, so the SPL can easily determine where to
load U-Boot proper from.
Factor out reading this value, as it will come in handy again shortly.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>