arch/sh: allow building in big-endian mode

arch/sh: allow building in big-endian mode

The SuperH architecture allows to be run in either little or big
endian mode. Some SuperH SoCs get the little vs. big endian decision
through mode pins sampled at reset, so if big endian has been choosen
by HW designers, it cannot be easily changed.

Therefore, it makes sense to allow building U-Boot for SuperH in big
endian mode. To allow this, the only change needed is to adjust the
OUTPUT_FORMAT() in the linker script.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>