Make sure that our version of certain utility function implementations gets used in the firmware.

firmware/include/utility.h

Index: firmware/include/utility.h
diff --git a/firmware/include/utility.h b/firmware/include/utility.h
index 0e05ee3be03a384daf97357b88aafe873ce52110..a412fd8251771b50588d0dc4fe618351e9acd4cf 100644
--- a/firmware/include/utility.h
+++ b/firmware/include/utility.h
@@ -54,6 +54,10 @@ int Memcmp(const void* src1, const void* src2, size_t n);
 /* Copy [n] bytes from [src] to [dest]. */
 void* Memcpy(void* dest, const void* src, uint64_t n);
 
+
+/* Implementations of the functions below must be built as part of the firmware
+ * and defined in lib/utility.c */
+
 /* Set [n] bytes starting at [s] to [c]. */
 void* Memset(void *dest, const uint8_t c, uint64_t n);
 
@@ -72,5 +76,4 @@ int SafeMemcmp(const void* s1, const void* s2, size_t n);
 #define memset _do_not_use_standard_memset
 #endif
 
-
 #endif  /* VBOOT_REFERENCE_UTILITY_H_ */