bootm: vxworks: Make do_bootm_vxworks() non-static

For future extension to other architectures, make do_bootm_vxworks()
a non-static function.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/common/bootm_os.c b/common/bootm_os.c
index da938c5..473b940 100644
--- a/common/bootm_os.c
+++ b/common/bootm_os.c
@@ -317,8 +317,8 @@
 	puts("## vxWorks terminated\n");
 }
 
-static int do_bootm_vxworks(int flag, int argc, char * const argv[],
-			     bootm_headers_t *images)
+int do_bootm_vxworks(int flag, int argc, char * const argv[],
+		     bootm_headers_t *images)
 {
 	if (flag != BOOTM_STATE_OS_GO)
 		return 0;