The Arduino IDE directory structure

• BASE: /usr/local/arduino or /usr/share/arduino
•     (On windows, typically C:\Program Files\Arduino)
   
• --  hardware/arduino  -  contains 'boards.txt'
•    +-----  cores
•             +-----  arduino  -  built-in
•             +-----  robot      -  built-in
•             +-----  xmega    -  added (symlink or copy)
•    +-----  variants
•             +-----  arduino         -  built-in
•             +-----  mega             -  built-in
•               etc.
•             +-----  xmega64d4    -  added (symlink or copy)
•             +-----  xmega32e5    -  added (symlink or copy)
•             +-----  xmega128a1  -  added (symlink or copy)
•               etc.