cpu/sam3x8e: added interrupt mode to uart driver
board/arduino-due: added missing UART defines

- added port defines to periph_conf.h
- added feature to Makefile.features
board/udoo: added missing UART defines

- added defines to periph_conf.h
- added UART feature to Makefile.features
cpu/sam3x8e: made UART0 work by editing syscalls
Merge pull request #1482 from haukepetersen/fix_sam3x_finished_uart

cpu/sam3x: completed low-level UART driver