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.
rpl: mute RPL

Removes all but one occurrence of printf and puts within routing (the
remaining puts is called from a debug function.
add backspace functionality in shell
Add 0x7f as backspace key in shell
native: -DNATIVE_INCLUDES if using NATIVEINCLUDES

It can be useful to know if the current compilation builds the native
board module, because e.g. then we must not include `sys/types.h`.

This diff adds `-DNATIVE_INCLUDES` to the make variable
`NATIVEINCLUDES`.
 core: Provide ssize_t in a common place

 Now you can include `kernel_types.h` if you need the `ssize_t`, without
 facing problems due to the terrible MSP toolchain.
stm32f0: be UART0 aware
stm32f4: be UART0 aware
stm32f3: be UART0 aware
stm32f1: unify UART0 use
stm32f*: rename STDIO buffer and enable getchar with UART0
Merge pull request #1471 from thomaseichinger/impl_uart0

cpu: make stm32f{0,1,3,4} comply to UART0 module
boards:mega2560: add debug and debug-server targets
Merge pull request #1696 from N8Fear/mega2560-debug

boards:mega2560: add debug and debug-server targets
drivers: import PIR motion sensor driver

Includes an application for manual testing.
Merge pull request #1714 from LudwigOrtmann/add_driver_pir

drivers: import PIR motion sensor driver
make: add objdump target
make: .PHONY missing targets from Makefile.include
Merge pull request #1723 from LudwigOrtmann/issue-1719

make: add objdump target + .PHONY missing targets from Makefile.include
Merge pull request #1698 from Kijewski/ssize_t-for-all

core: Provide ssize_t in a common place