boards/chronos: clean up display driver a bit

reduce scope of variables
remove useless comments and spaces

saves 2 bytes =)
native: increase rtc realism

Prevent rtc enabling when not initialized.
Merge pull request #1474 from LudwigOrtmann/chronos_cleanup_display

boards/chronos: clean up display driver a bit
Merge pull request #1477 from LudwigOrtmann/native_rtc_increase_realism

native: increase rtc realism
cpu - stm32f4: add support for stm32f415rg
cpu - stm32f4: added support for more channels/devices in periph impls
cpu - stm32f4: removed reference to non existing .h file in stm32f415xx.h
board - msbiot: initial import for the msbiot
added changes of improved UART driver interface
added changes of improved GPIO driver interface