buildtest: retry failed builds once

On Travis CI often builds fail spuriously.

This PR lets `make buildtest` retry the build once.
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
Merge pull request #1546 from Kijewski/buildtest-retry

buildtest: retry failed builds once
drivers: updated low-level PWM driver interface
cpu: adjusted pwm driver impl for lpc2387
Merge pull request #1466 from haukepetersen/fix_periph_pwmif

drivers: improvements to the low-level pwm driver interface
board/cpu: added pwm driver for stm32f4discovery
tests: added test for low-level PWM driver
Merge pull request #1467 from haukepetersen/add_stm32f4_pwm

cpu: added pwm driver for the stm32f4
board/cpu: added SPI driver for stm32f0discovery
[SQUASH ME] finished 0.1 of spi driver
SPI: more fixes including DEBUG info
cpu: mini cleanup to spi driver
tests: added periph_spi test
workin spi master/slave for stm32f0discovery
fixed problem with slaves response
Merge branch 'add_stm32f0_spi' of https://github.com/PeterKietzmann/haukepetersen-RIOT into add_stm32f0_spi