armcm_link: Rename CONFIG_FLASH_START to CONFIG_FLASH_APPLICATION_ADDRESS

Rename the build symbol name for better clarity on what it represents.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/lpc176x/main.c

@@ -44,7 +44,7 @@ DECL_INIT(watchdog_init);
 void
 bootloader_request(void)
 {
-    if (!CONFIG_FLASH_START)
+    if (!CONFIG_FLASH_APPLICATION_ADDRESS)
         return;
     try_request_canboot();
     // Disable USB and pause for 5ms so host recognizes a disconnect