Generate hex image for all targets by default

Mbed OS builds .bin images by default for most targets, but many
targets (e.g. ST targets running the latest ST-LINK firmware) also
support .hex files, making this demo much easier to prepare. In
case a target doesn't support .hex files, README.md already contains
steps for merging .hex files into a single one and converting it to
a .bin file.

Author: LDong-Arm

mbed_app.json

@@ -15,6 +15,7 @@
         "*": {
             "mcuboot.bootloader-build": 0,
             "target.c_lib": "small",
+            "target.OUTPUT_EXT": "hex",
             "mcuboot.log-level": "MCUBOOT_LOG_LEVEL_DEBUG",
             "mbed-trace.enable": true
         },