Allow more avr variants with build_marlin*

buildroot/bin/build_marlin

 #!/usr/bin/env bash
 
-arduino --verify --board arduino:avr:mega:cpu=atmega2560 Marlin/Marlin.ino
+case "$#" in
+  0 ) BOARD=arduino:avr:mega:cpu=atmega2560 ;;
+  * ) BOARD=arduino:avr:$1 ;;
+esac
+
+arduino --verify --board $BOARD Marlin/Marlin.ino

buildroot/bin/build_marlin_fail

 #!/usr/bin/env bash
 
-if arduino --verify --board arduino:avr:mega:cpu=atmega2560 Marlin/Marlin.ino ; then
-  return 1
-else
-  return 0
-fi
+build_marlin $@ && exit 0
+exit 1