unittests: make: Fix unittests for branch changes

Changing between branches in git typically leaves directories empty and
does not remove them. If you have different unittests in different
branches this leads to an error in building them, since the current
configuration expects a Makefile in all `tests-*` directory. This change
configures Make to search for a Makefile in `tests-*` rather than just
assuming that there is one.
add backspace functionality in shell
Add 0x7f as backspace key in shell
Merge pull request #1653 from medicalwei/shell_backspace

Add backspace functionality in shell
board/iot-lab_M3: cleaned up SPI in periph_conf.h
cpu/stm32f1: cleanup in SPI driver

- made config more versatile
- added pin configuration to spi_init()
board/iot-lab_M3: cleanup of AT86RF231 driver

- removed board specific code
- added interface definitions to board.h
drivers/at86rf231: rework and clean-up

- moved all board specific code to low-level driver calls
- moved some functions from the board dir into at86rf231.c
- added doxygen headers
cpu/stm32f1: added clock config to cpu.c
board/iot-lab_M3: added clock config to periph_conf
Merge pull request #1735 from haukepetersen/fix_stm32f1_somecleanup2

cpu/board: cleanup for clock init on iot-lab_M3
Merge pull request #1736 from haukepetersen/fix_stm32f1_somecleanup

board/iot-lab_M3: cleanup of SPI and radio driver
initial import of stm32f4discovery spi low-level driver
Merge pull request #1464 from PeterKietzmann/stm32f4_spi_driver

Initial Stm32f4 spi driver
Initialize addresses from CPU ID
auto_init: Put net_if auto-initialization into its own function
Merge pull request #837 from authmillenon/auto-init-addresses

auto_init: Initialize link-layer addresses from CPU ID
reverted forcing travis to install `gcc-arm-none-eabi=4-*`
Merge pull request #1742 from BytesGalore/revert_workaround_for_gcc_arm_none

Travis: reverted forcing travis to install `gcc-arm-none-eabi=4-*`
travis: remove comment obsoleted by #1742