drivers: Initial import of SPI low-level driver IF
drivers: fixes to the SPI low-level interface
driver: updated SPI driver interface
[SQUASH ME] added fixed speed values
[SQUASH ME] removed UNDEFINED from device list
[SQUASH ME] fixed signature of spi_transfer_reg()
[SQUASH ME] added begin_transaction, fixed doc
make: force traversal into PKG's, same as normal modules
pkg: add USEPKG module "libfixmath"

This PR adds the USEPKG module "libfixmath".
It integrates https://code.google.com/p/libfixmath/ into RIOT, cmp. #1457.
crypto: get rid of compiler warnings
Merge pull request #1460 from Kijewski/issue-1457

pkg: add USEPKG module "libfixmath"
sixlowpan: decrement processing semaphore when discarding
sixlowpan: some additional debugging
Merge pull request #1523 from OlegHahm/sixlowpan_minor

sixlowpan: minor cleanups and debugging
make: provide BOARD, MCU and CPU name as a string

This PR adds the defined `RIOT_BOARD`, `RIOT_CPU` and `RIOT_MCU`.
They each are defined as `BOARD_${BOARD}`, `CPU_${CPU}`, and
`MCU_${CPU}`, resp.

`BOARD_${BOARD}` is defined as the string `"${BOARD}"`, but not in upper
case. Same for the other two.
Merge pull request #1524 from Kijewski/make-board-name-string

make: provide BOARD, MCU and CPU name as a string
core: harmonizes the data type for the process ID

Instead of using differing integer types use kernel_pid_t for process
identifier. This type is introduced in a new header file to avoid
circular dependencies.
ccnl: pass relay_pid as pointer
at86rf231: use radio_packet_length_t
cc110x: use radio_packet_length_t