blackfin: x86: bootm: Handle PREP stage of bootm

The OS function is now always called with the PREP stage. Adjust the
remaining bootm OS functions to deal with this correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/lib/bootm.c b/arch/x86/lib/bootm.c
index 0d3250c..b84e35a 100644
--- a/arch/x86/lib/bootm.c
+++ b/arch/x86/lib/bootm.c
@@ -48,6 +48,8 @@
 	size_t		len;
 #endif
 
+	if (flag & BOOTM_STATE_OS_PREP)
+		return 0;
 	if ((flag != 0) && (flag != BOOTM_STATE_OS_GO))
 		return 1;