spl: spl_nor: Move legacy image loading into spl_legacy.c

spl: spl_nor: Move legacy image loading into spl_legacy.c

Move the legacy image loading into spl_legacy.c. This makes it easier
to extend the legacy image handling with new features that other
SPL loaders might use (e.g. spl_spi.c etc).

No functional change intended.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Weijie Gao <weijie.gao@mediatek.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>