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
boards/chronos: clean up display driver a bit

reduce scope of variables
remove useless comments and spaces

saves 2 bytes =)
make: fix buildsizes* targets

also ignore BOARD_INSUFFICIENT_RAM boards
unify/make work BOARD filter
native: increase rtc realism

Prevent rtc enabling when not initialized.
drivers: some improvements to the ADC driver IF

- removed UNDEFINED value from device enum
- added pre-defined precision values
- added power[on|off] functions
- some minor doxygen fixes
boards/arduino-due: some cleanup

- added parenthesis for LED macros
- changed mapping for LED_* to LED_RED_*
- removed some old values from periph_conf.h
boards/udoo: cleanup in periph_conf.h

- removed unused defines
Merge pull request #1479 from haukepetersen/fix_sam3x_minorfixes

board - arduino-due: some cleanup
Merge pull request #1469 from haukepetersen/fix_periph_adcif

drivers: some improvements to the ADC driver interface
ringbuffer: remove example
ringbuffer: add convenience functions

This patch add `peek` functionality, and empty and full helpers.
ringbuffer: fix parameter types

It is a bad idea to use signed types for lengths.
Mark pointers a `restrict`, since the ringbuffer is not thread safe
anyway.
ringbuffer: remove pointer to the end

There is no need for an explicit pointer to the end of the buffer.
ringbuffer: don't overwrite in `ringbuffer_add`