- moved definition of CPUID_LEN to periph_cpu.h
- fixed some doxygen
- simplyfied some implementations

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

* removed RIOT unrelated cpu documentation
* introduced cpu_specific prefix to mark such documentation
* put oneway_malloc and crypto into sys group

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