add BOOT_LOAD_PIN, update BL from SRAM and Jeff Probe Board

[hlyi]

This request to add the following features and board:

• Add "Jeff Probe" board.
• Add "BOOT_LOAD_PIN".
  • The code was based upon SAM-BA. when boots, pulling BOOT_LOAD_PIN low would enter bootloader mode.
  • The define can be set in board_config.h
  • This feature costs about ~36 Bytes.
• Add ability to update bootloader in UF2 mode without touching application flash
  • The feature can be enable by adding "USE_SRAM_BL_FLASH = 1" in "board.mk" file.
  • To enable this feature for the first time, add "CONV_SRAM_BL=1" option to make command. The "update-bootloader-.uf2" will still overwrite application flash
  • For subsequent updates, do not use "CONV_SRAM_BL=1" option, the "update-bootloader-.uf2" will run from SRAM, and will not overwrite application flash.
  • This feature costs about ~108 Bytes.