Fix for mega as target device.
Fix for mega as target device.

The conditional include of HardwareSerial.cpp allows targeting the mega
board.:
a

etk29321/firmware,etk29321/firmware,BrewPi/firmware,glibersat/firmware,glibersat/firmware,glibersat/firmware,BrewPi/firmware,etk29321/firmware,glibersat/firmware,glibersat/firmware,BrewPi/firmware,glibersat/firmware,etk29321/firmware,BrewPi/firmware,glibersat/firmware,BrewPi/firmware,BrewPi/firmware,BrewPi/firmware,etk29321/firmware,etk29321/firmware,BrewPi/firmware