crypto: get rid of compiler warnings
core: moved definition of KERNEL_PID_NULL

kernel_pid_t is defined in kernel_types.h, thus it makes sense to move
the macro for an invalid kernel pid there, too.
net: fix kernel_pid_t checks

KERNEL_PID_NULL is a negative number. If kernel_pid_t variables are
initialized to this value, one have to check for lt/gte 0.
core: renamed KERNEL_PID_NULL to KERNEL_PID_UNDEF

As @authmillenon pointed out the "null" in the old name is somewhat
misleading, since the actual value is -1.
initialize kernel_pid_t correctly
make kernel_pid_t comparisons consistent
ccnl: duplicate variable names

Some variables had multiple definitions, but could be removed or
declared with a smaller scope.
net: changed name of internal variable

To avoid naming conflicts + made variable static and volatile as it
should be.
tests: combine declaration and definition
fixed remaining variables to kernel_pid_t
Merge pull request #1549 from OlegHahm/kernel_pid_fixup

Handle kernel_pid_t consistently
drivers: improvements to UART driver interface

- removed UNDEFINED value from device enum
- added argument for callbacks
- add power[on|off] functions
- removed tx_end() function in favor of cb return value
make: Add target to show if boards that are supported by apps
[SQUASH ME] make: move info-boards-supported to Makefile.buildtests
make: Add info prefix for non-building targets in Makefile.buildtests
[SQUASH ME] make: Add info prefix for non-building targets in Makefile.buildtests
board/cpu: adjusted uart driver implementations

for
- sam3x8e
- stm32f0
- stm32f4
- sam3x8e
- nrf51822
tests/periph_uart: added two UART driver tests

- test for UART driver in blocking mode
- test for UART driver in interrupt driven mode
Merge pull request #1481 from haukepetersen/fix_drivers_uartif

drivers: improved UART driver interface
Travis: auto rebase on master before running

It can happen that two PRs don't interfere with each other in the sense
that they hurt automatic rebasing, but still do not work together. The
prime example is PR1 changes the API of some function that PR2 uses. If
PR1 is merged, and PR2 is not rebased before merging, the error might
get unnoticed before the next build. Travis CI would have have told both
PR1 and PR2 are fine, but will complain (rather unnoticedly) that our
master does not compile.

This PR automatically rebases the PR on top of the current master,
before running the tests. If the automatic rebase fails, then this is
fine, because you will need to manually rebase again before merging,
anyway. The manual rebase, i.e. new push, will trigger Travis CI.

So, the main idea of this PR is that to can hit the "Restart Build"
button in Travis CI before hitting the merge button in Github.