Add detect macros for AVR XMega MCU.

Hardware interrupts should be enabled separately by using the lines:
  #include <avr/io.h>
  PMIC.CTRL |= PMIC_LOLVLEX_bm | PMIC_MEDLVLEX_bm | PMIC_HILVLEX_bm;
These should go into a separate init file specific for AVR XMega.

Signed-off-by: Onno <developer@gorgoz.org>
Reviewed-by: Luca Ottaviano <lottaviano@develer.com>
git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4843 38d2e660-2303-0410-9eaa-f027e97ec537