spl: Extract legacy image handling into separate file

spl: Extract legacy image handling into separate file

This patch moves the legacy image handling into a separate file, which
will be extended with other legacy image features later.

No function change intended.

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