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
Merge pull request #72 from mattande/makefile_program

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