Add factory reset request

[hathach]

Is your feature request related to a problem? Please describe.

• Add back Factory reset: clear all nvm + application + bootloader setting (leaving only bootloader + mbr + sd), this is still useful feature to troubleshoot user issue.
• factory reset can be triggered by:
  • dedicated pin on boot with feather nrf52832 (need to specifically enabled)
  • magic number with DFU_MAGIC_FACTORY_RESET to activate from simple app/sketch
• revert changes in 99d31d2