BUNDLED_AVR_TOOLS_DIR is now set correctly

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) without having to install the whole arduino IDE, e.g. raspberry pi.

Previously BUNDLED_AVR_TOOLS_DIR was set to an empty string (that's what ?= does for unset variables) as the directory doesn't exist, which meant that "ifdef BUNDLED_AVR_TOOLS_DIR..." was set to empty, 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.