vboot2: Add stub implementation for vb2ex_printf()

BUG=none
BRANCH=ToT
TEST=manual

  make VBOOT2=1 DEBUG=1 runtests

Change-Id: I5e99082d713e2f8ad2c56a10b86d0e0a44037549
Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/243360
Reviewed-by: Randall Spangler <rspangler@chromium.org>
diff --git a/firmware/2lib/2stub.c b/firmware/2lib/2stub.c
index 375d4fa..971286c 100644
--- a/firmware/2lib/2stub.c
+++ b/firmware/2lib/2stub.c
@@ -5,10 +5,23 @@
  * Stub API implementations which should be implemented by the caller.
  */
 
+#include <stdarg.h>
+#include <stdio.h>
+
 #include "2sysincludes.h"
 #include "2api.h"
 
 __attribute__((weak))
+void vb2ex_printf(const char *func, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	fprintf(stderr, "%s: ", func);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+}
+
+__attribute__((weak))
 int vb2ex_tpm_clear_owner(struct vb2_context *ctx)
 {
 	return VB2_ERROR_EX_TPM_CLEAR_OWNER_UNIMPLEMENTED;