cpu/sam3x8e: fixes to timer driver

- renamed config variable
- placed thread_yield() correctly
cpu/sam3x8e: put PendSV IRQ to lowest prio
cpu/sam3x8e: added output to isr_hard_fault
cpu/sam3x8e: fixed style in startup.c
Merge pull request #1825 from haukepetersen/fix_sam3x_misc

cpu/sam3x8e: general fixes