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>
diff --git a/common/spl/spl_nor.c b/common/spl/spl_nor.c
index 5270401..68c1241 100644
--- a/common/spl/spl_nor.c
+++ b/common/spl/spl_nor.c
@@ -35,7 +35,7 @@
 	 */
 	spl_image->flags |= SPL_COPY_PAYLOAD_ONLY;
 
-#ifdef CONFIG_SPL_OS_BOOT
+#if CONFIG_IS_ENABLED(OS_BOOT)
 	if (!spl_start_uboot()) {
 		/*
 		 * Load Linux from its location in NOR flash to its defined