spl: Drop the switch() statement for OS selection

This code is pretty ugly, with many #ifdefs

There are quite a lot of IH_OS_U_BOOT values so the compiler struggles
to create a jump table here. Also, most of the options are normally
disabled.

Change it to an else...if construct instead. Add an accessor for the
spl_image field behind an #ifdef to avoid needing #ifdef in the C code.

Signed-off-by: Simon Glass <sjg@chromium.org>