Copy some files from cpu/sam3x8e to cpu/cc2538.
Add the latest cc2538-bsl script from https://github.com/JelmerT/cc2538-bsl/commit/f3701a0
Add support for the Texas Instruments CC2538 ARM Cortex-M3 MCU and developer kit.
Merge pull request #1697 from locicontrols/cc2538

Add support for the Texas Instruments CC2538 ARM Cortex-M3 MCU and developer kit.
tests/periph_spi: let build fail without SPI
make: introduce and use periph_spi feature
documentation: added description for packet queue
core: refactor check for valid pid

move into header and remove redundant condition
core: add doxygen header for kernel_types
core/msg: add DEVELHELP checks for valid pid
Merge pull request #1810 from OlegHahm/pktqueue_doxygen_group_brief

documentation: added description for packet queue
Merge pull request #1794 from LudwigOrtmann/valid_pid

core: refactor check for valid pid
Merge pull request #1809 from LudwigOrtmann/Makefile.features-SPI

make: periph_spi feature
Initial import of nrf24l01p netdev implementation
[SQUASH ME] addressed latest changes from lowlevel driver
[SQUASH ME] rebasen and added periph_spi feature
[SQUASH ME] changed protocol type name to NETDEV_PROTO_NRV24L01X
[SQUASH ME] added nrf24l01p transceiver module to default.h