The list of board symbols can be generated by this shell command:
$ grep board= boards.txt | cut -f2 -d= | sort -u
AVR_ADK
AVR_BT
AVR_DUEMILANOVE
AVR_ESPLORA
AVR_ETHERNET
AVR_FIO
AVR_GEMMA
AVR_LEONARDO
AVR_LILYPAD
AVR_LILYPAD_USB
AVR_MEGA
AVR_MEGA2560
AVR_MICRO
AVR_MINI
AVR_NANO
AVR_NG
AVR_PRO
AVR_ROBOT_CONTROL
AVR_ROBOT_MOTOR
AVR_UNO
AVR_YUN
The boards are defined by the "build.board" property in the boards.txtboards.txt file.
mini.build.board=AVR_MINI
This property is used by the build reciperecipe together with ARDUINO_-prefix.
-DARDUINO_{build.board}
Cheers!