- removed ISR_STACKSIZE define where unused (set to 0)
- removed thread_arch_isr_stack_usage(), thread_arch_isr_stack_start(),
  and/or thread_arch_isr_stack_pointer() where not implemented

add atmega328p support with:
uart, timer, spi and gpio

Fix cpu/lpc2387/include/lpc2387.h

As discussed in #2725, this commit renames a number of stacksize constants to
better convey their intended usage. In addition, constants for thread priority
are given a `THREAD_` prefix. Changes are:

* KERNEL_CONF_STACKSIZE_PRINTF renamed to THREAD_EXTRA_STACKSIZE_PRINTF
* KERNEL_CONF_STACKSIZE_DEFAULT renamed to THREAD_STACKSIZE_DEFAULT
* KERNEL_CONF_STACKSIZE_IDLE renamed to THREAD_STACKSIZE_IDLE
* KERNEL_CONF_STACKSIZE_MAIN renamed to THREAD_STACKSIZE_MAIN
* Move thread stacksizes from kernel.h to thread.h, since the prefix changed
* PRIORITY_MIN renamed to THREAD_PRIORITY_MIN
* PRIORITY_IDLE renamed to THREAD_PRIORITY_IDLE
* PRIORITY_MAIN renamed to THREAD_PRIORITY_MAIN
* Move thread priorities from kernel.h to thread.h since the prefix has changed
* MINIMUM_STACK_SIZE renamed to THREAD_STACKSIZE_MINIMUM for consistency

fixes #1190

- fixed spelling in uart driver
- marked cortexm-common a phony target
- fixed Makefiles
- added reboot interface implementation
- fixed documentation

- added UART0_BUFSIZE define

- added license header

- fixed documentation issues