Spark firmware v0.2.1

Highlights

• OTA reliability improvements
  (core-firmware,
  core-common-lib,
  core-communication-lib)
• Allow Spark.publish inside Spark.function
  (core-communication-lib)
• Add Network.ping() (core-firmware)
• Enable factory reset from firmware (only on new bootloader)
  (core-common-lib,
  bootloader)