Document intentional fallthroughs + fix harmless unintended ones

Change-Id: I1ca5b1027ec30cbf61093bab35b980196ed14e6b

firmware/common/file_internal.c

@@ -647,6 +647,7 @@ int open_stream_internal(const char *path, unsigned int callflags,
     {
     case WALK_RC_FOUND_ROOT:
         IF_MV( rc = rootrc; )
+        /* fallthrough */
     case WALK_RC_NOT_FOUND:
     case WALK_RC_FOUND:
         /* FF_PROBE leaves nothing for caller to clean up */

firmware/common/vuprintf.c

@@ -753,6 +753,7 @@ static int format_double_radix(double f,
         if (prec_rem) {
             prec_rem--;
         }
+        /* fallthrough */
     case 1: /* %e, %E */
         explen = 2;
         break;