BUNDLED_AVR_TOOLS_DIR is now set correctly using := instead of ?=,
so that installations using only the arduino-core packages that
don't have the $(ARDUINO_DIR)/hardware/tools/avr directory can
still use the avr-g++ tools found in the $PATH (/usr/bin)

Previously BUNDLED_AVR_TOOLS_DIR was set to an empty string as
the directory doesn't exist, which meant that "ifdef BUNDLED_AVR_TOOLS_DIR..."
was set rather than skipping to "else SYSTEMPATH_AVR_TOOLS_DIR...."

No user would set BUNDLED_AVR_TOOLS_DIR so the assignment operator
:= should be used not ?=

Updated the version info/changes in various locations.