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
Fixing unit test build process