Added support for (new) Atmel SAMD21 XPlained board

The board at hand has chip with revision=0x1
Fix system crash when using bossac on UART

The crash is caused by an attempt to write to flash address space
(0x804000). The address has been changed to the middle of SRAM
(0x20004000). This bug does not show up when using USB CDC.
Merge pull request #6 from 8devices/arduino

Fix crash for SAMD and add support for SAMD21J18A.