Fix wrong variables used for custom bootloader programming

[lippertmarkus]

Changes:

• As in the boards.txt (e.g. for the uno: https://github.com/platformio/platform-atmelavr/blob/de57dda3eb6193d19ab51136d3347b8e56933a00/boards/miniatmega328.json#L9-L15) lfuse, hfuse and efuse must be used instead of low_fuses, high_fuses and extended_fuses. Else running platformio run -t bootloader will fail with "Missing bootloader fuses!" - see thread: https://community.platformio.org/t/bootloader-target-fails-with-missing-bootloader-fuses-although-set/11684
• Removed GitHub link pointing to a non-existent repository.

[valeros]

Many thanks!