Updated docs - added BOOTLOADER_PARENT

To use a bootloader that's not in the default location, you have to set `BOOTLOADER_PARENT` otherwise `BOOTLOADER_PATH` and `BOOTLOADER_FILE` are appended to the default `/usr/share/arduino/hardware/arduino/bootloaders`

So I added the user-definable `BOOTLOADER_PARENT` to the docs, with a note about it being a dependancy of setting the other 2 variables.

I also removed the absolute path example from `BOOTLOADER_PATH` that doesn't work