fixes in makefile to ensure successfull build on linux plateform

I was trying to build a brand new makefile to enable linux build for brewpi-avr, until I realized it has already been done in the dynaconfig branch by matthew!

So I made some really small fixes ("/" instead of "\", obj dir & final dir creation in recipes) to have the current makefile running on linux

I tested it with the command :
make ARDUINO_HOME=/usr/share/arduino ARDUINO_VERSION=103 BOARD=uno

which matches my setup