Remove redundant board.mcu in boards.txt, fix precompiled libs (#382)

* Remove redundant/wrong second board.mcu in boards.txt

Fixes #380

Boards.txt had the setting board.mcu twice.  The first one was correct,
while the second one wasn't and shouldn't have been there.  Remove it.

* Include fix from @rei-vilo in #381

Author: earlephilhower