* define weak symbol for ${irq_name}_isr
* put them in the rom vector table
* declare reset_vector weak in case someone wants to insert code there
* remove null_handler (null handlers should be only requested
explicitly, and finding out why the chip gets stuck is easier than
determining whether or not a no-op interrupt handler was executed)
includes minor refactoring in example code and modification of how the
generic and the tinygecko specific vector.h go together (bringing it in
line with stm32/f1's memorymap.h)
