Implemented GPS DateTime feature

With this patch we're able to get the date from the GPS (Mediatek and Ublox).
Changes:
Added _mktime to gps-library to convert from utc to linux time format.
_mktime is directly from the standard c-library, not written by me.

Added getting UTC times to Mediatek and Ublox

The converted (unix epoch) datetime is transmitted to the user using the SYSTEM_TIME (and only this) message.

Discussion: https://groups.google.com/forum/#!topic/drones-discuss/m-yEWsInRp0

Signed-off-by: Simon Egli <simon.egli@tik.ee.ethz.ch>