Add makefile targets for programming via dfu-utils and cloud API

 * Add make target 'program-dfu' which invokes dfu-utils to program
   the current core-firmware.bin file to STM32 flash
 * Add make target 'program-cloud' which invokes curl to program
   the current core-firmware.bin file to STM32 flash using the
   cloud API. This requires the core's ID and access_token in the
   environment, either by:
```
   $ SPARK_CORE_ID=xxx SPARK_ACCESS_TOKEN=xxx make program-cloud
```
   Or exported in the environment previously:
```
   $ export SPARK_CORE_ID=xxx
   $ export SPARK_ACCESS_TOKEN=xxx
   $ make program-cloud
```