vboot_reference: fix unittest when building with clang.

When linking vboot_api_kernel4_tests, there are two VbBootNormal()
available, the gcc chooses the one in vboot_api_kernel4_tests.c and
the test passes, the clang chooses the one in vboot_api_kernel.c and
make the unittest fail. This CL makes the one in vboot_api_kernel.c
a weak symbol so that clang can choose the one in
vboot_api_kernel4_tests.c

BUG=chromium:498469
BRANCH=none
TEST=CC=x86_64-cros-linux-gnu-clang  FEATURES='test'
     emerge-amd64-generic vboot_reference

Change-Id: Ibcb78ee055fc9485dbc2bcc1d1cf98144a1a3b64
Reviewed-on: https://chromium-review.googlesource.com/276504
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>

firmware/include/vboot_api.h

@@ -36,6 +36,11 @@
  */
 typedef uint32_t VbError_t;
 
+/*
+ * Define test_mockable for mocking functions.
+ */
+#define test_mockable __attribute__((weak))
+
 /*
  * Predefined error numbers.  Success is 0.  Errors are non-zero, but differ
  * between functions.  For example, the TPM functions may pass through TPM