[warnings] fix a few warnings introduced with newer version of gcc

Most of the warnings are new, such as needing to mark fallthroughs on
cases explicitly. A few are based on signed vs unsigned comparisons.

Disable one warning that was annoying about comparing null to arguments
marked nonnull.

app/lkboot/lkboot.c

@@ -112,6 +112,7 @@ static int lkb_send(lkb_t *lkb, u8 opcode, const void *data, size_t len) {
             break;
         }
         len = 0;
+        // fallthrough
     default:
         lkb->state = STATE_ERROR;
         opcode = MSG_FAIL;

app/lpcboot/lpcboot.c

@@ -251,6 +251,7 @@ void handle(u32 magic, u32 cmd, u32 arg) {
 		return;
 	case 'A':
 		boot_app();
+		/* fallthrough */
 	case 'R':
 		/* reboot "normally" */
 		reply[1] = 0;

app/tests/float_test_vec.c

@@ -69,7 +69,7 @@ static const union double_int float_test_vec[] = {
 };
 
 #define countof(a) (sizeof(a) / sizeof((a)[0]))
-static const unsigned int float_test_vec_size = countof(float_test_vec);
+__attribute__((unused)) static const unsigned int float_test_vec_size = countof(float_test_vec);
 
 #define PRINT_FLOAT \
         printf("0x%016llx %f %F %a %A\n", \