cores: Adds missing external interrupt for m16/m32

 > Async external interrupt (INT2) was missing on m16/32. Since unlike the others
   this interrupt uses only 1 bit for sense control avr_extint.* were modified to
   support this new mode.
Merge pull request #101 from dougszumski/mx_core_add_extint

cores: Adds missing external interrupt for m16/m32