spl: LOAD_FIT_FULL: Do not hard-code os to IH_OS_U_BOOT

The information on the OS should be contained in the FIT, as the
self-explanatory "os" property of a node under /images. Hard-coding
this to U_BOOT might send us down the wrong path later in the boot
process.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>