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

firmware/lib/vboot_api_kernel.c

@@ -238,7 +238,8 @@ int VbUserConfirms(VbCommonParams *cparams, uint32_t confirm_flags)
 	return -1;
 }
 
-VbError_t VbBootNormal(VbCommonParams *cparams, LoadKernelParams *p)
+VbError_t test_mockable
+VbBootNormal(VbCommonParams *cparams, LoadKernelParams *p)
 {
 	/* Boot from fixed disk only */
 	VBDEBUG(("Entering %s()\n", __func__));

firmware/stub/vboot_api_stub_init.c

@@ -61,7 +61,7 @@ uint64_t VbExGetTimer(void)
 	return (uint64_t)tv.tv_sec * 1000000 + (uint64_t)tv.tv_usec;
 }
 
-VbError_t VbExNvStorageRead(uint8_t *buf)
+VbError_t test_mockable VbExNvStorageRead(uint8_t *buf)
 {
 	return VBERROR_SUCCESS;
 }