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/atsamd/main.c

@@ -14,7 +14,7 @@
 void
 bootloader_request(void)
 {
-    if (!CONFIG_FLASH_START)
+    if (!CONFIG_FLASH_APPLICATION_ADDRESS)
         return;
     // Bootloader hack
     irq_disable();