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_usb.c b/common/spl/spl_usb.c
index 3648de3..67d5030 100644
--- a/common/spl/spl_usb.c
+++ b/common/spl/spl_usb.c
@@ -47,7 +47,7 @@
 
 	debug("boot mode - FAT\n");
 
-#ifdef CONFIG_SPL_OS_BOOT
+#if CONFIG_IS_ENABLED(OS_BOOT)
 	if (spl_start_uboot() ||
 	    spl_load_image_fat_os(spl_image, stor_dev, partition))
 #endif