auto_init_gnrc_netif: set priorities one lower than 6LoWPAN

Refactor samd21

kw2xrf: set tx_power in gnrc_netdev_t

gnrc_rpl: adapt default parameters and expose to Makefile 

boards: add arduino uno and duemilanove support

atmega: use software interrupt for context swap

- blacklist arduino-uno and arduino-duemilanove for
  coap, libfixmath_unittests, lwip, nhdp,
  pthread, pthread_barrier, pthread_cleanup, pthread_condition_variable
  pthread_cooperation, pthread_rwlock and pthread_tls tests.

- fix sys/pipe build

- unittests: boards added to BOARD_INSUFICIENT_MEMORY list.

Uno and Duemilanove(atmega328p version) are nearly the same boards.
The only difference is that the Duemilanove use an FTDI usb chip,
while the Uno use an Atmel which acts as USB/Serial converter.
All of the code needed to support these boards is in arduino-common.

This define conflicts with LITTLE_ENDIAN defined in
include/machine/endian.h which is part of gcc-arm-none-eabi.

Also, the define does not seem to be used by the ASF included files.

Notice the extra fields in uart_conf_t for rx_pad and tx_pad.

The pinmux is now part of the board config. The pad setting is done with
clear names instead of numbers.

The board config can define SPI_1_EN as 0, and in that case #ifdef won't
work.

Add some more comments, and fix typos.

The former i2c_init_master was too complicated. It was trying to set IN/OUT
mode of the pins, but all that is needed is to set the proper MUX.

Also the configuration for the boards was incomplete (no MUX, no pad
setting).

It was tested on a SODAQ Autonomo, but not on a samr21-xpro

Notice that saml21 does not use I2C (yet).

A generic function is added to initialize a SERCOM.

Notice that uart_conf_t was expanded with pad settings, but it isn't used
yet.

In the Makefile.include of the board it is required to set the correct
define.

Currently only SAMRG18A is supported, as required by samr21-xpro.

Notice that RIOT was using FDPLL96M, which is indeed the name in the
datasheet. But Atmel is using GCLK_GENCTRL_SRC_FDPLL.

tests/driver_at86rf2xx: corrects flag compare

vagrant: minimize Vagrantfile and make it loadable

By parameterizing certain paths with `RIOTBASE`, it is possible to load
this Vagrantfile from other Vagrantfiles. This feature will be used by the
Tutorials repository, so that this Vagrantfile has no knowledge about
the tutorials, hence the deletion of the `tutorials` definition at this
place.

samd21: i2c: check all busstate bits

boards/nucleo-f207: Use cortex_m3_2 MCU_GROUP

minor typos in README corrected

vagrant: sync Tutorials folder into vbox

vagrant: prevent error by checking existence of .gitconfig

correcting linker LTO flags on OSX