spl: Make use of CONFIG_IS_ENABLED(OS_BOOT) in SPL/TPL common code paths

When building a system that has both TPL and SPL_OS_BOOT, code which
tests for CONFIG_SPL_OS_BOOT will be built and enabled in TPL, which is
not correct.  While there is no CONFIG_TPL_OS_BOOT symbol at this time
(and likely will not ever be) we can use CONFIG_IS_ENABLED(OS_BOOT) in
these common paths to ensure we only compile these parts in the SPL
case.

Signed-off-by: Tom Rini <trini@konsulko.com>