all over the place: use sched_active_pid

In many places we needlessly use `sched_active_thread->pid` whilst we
already have `sched_active_pid` with the same value, and one less
indirection.

`thread_getpid()` is made `static inline` so that there is no penalty in
using this function over accessing `sched_active_pid` directly.
cpu: initial import of stm32f1
boards: initial import of iot-lab_M3
at86rf231: introduce variable to keep internal driver state

The at86rf231 radio transceiver provides one IRQ (TRX_END) signaling
end of transmission or reception but no way to distinguish between
these.
stm32f1: implement missing UART0 parts
iot-lab_M3: refactor at86rf231 driver
stm32f1: implement SPI register access functions
at86rf231: handle rx tx state changes correctly
stm32f1: implement new cpuid_get
stm32f1: SPI: implement speed setting properly
Implement a minimal RTC interface
Merge pull request #1020 from thomaseichinger/real_rtc_interface

periph_drivers: Introduce a minimal RTC interface
Merge pull request #1363 from thomaseichinger/thirdparty_boards_import

board: cpu: initial import of iot-lab_M3
Merge pull request #1575 from Kijewski/use-sched_active_pid

all over the place: use sched_active_pid
periph: added a random-data driver interface
Merge pull request #939 from mehlis/random-driver

random: added a random driver interface
cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist board arduino-mega2560 for tests/examples that don't build