driver_periph: minor documentation fix
x86: disable `--gc-sections` for Travis' sake

I could not reproduce the problem at home, but on Travis CI after
merging #1415 tests/unittest failed to execute for qemu-i386.

There is a crash early in the initialization, caused by a #PF. The
execution hangs afterwards (`cli; 0: hlt; jmp 1b`), and Travis kills
the execution after 10 minutes.
Merge pull request #1551 from authmillenon/fix-periph-doc

driver_periph: minor documentation fix
Merge pull request #1552 from Kijewski/fixup-1415

x86: disable `--gc-sections` for Travis' sake
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.
SQUASH ME: stupid mistake