2017q2 release

This release covers everything from the 2017q1 release on 2017-04-05 and has been
tested with avr-gcc v5.4.0 from Atmel and arm-none-eabi-gcc 2017q2 from ARM.

Breaking changes:

• No breaking changes

Major features:

• STM32L4 target support and drivers
• Improve assert implementation and make use of them

Major fixes:

• Fix issues and warnings in release tests
• Fix wrong ns_per_loop rounding in board definitions

Known bugs:

• xpcc may generate separate IRQ handlers for shared interrupts. See #88.
• GPIO connect on STM32F1 is still broken. See #178 for discussion.
  The API from MODM will not be backported to xpcc however.
• STM32L0/L4 hardware I2C driver has limitations on restart behaviors. See #255.

New development board targets:

• NUCLEO-L476RG as nucleo_l476rg
• STM32L476G-DISCO as stm32l476_discovery

New device drivers:

• DS1302
• TCS3472X

Many thanks to all our contributors of this release.
A special shoutout to first timers (🎉🎊):

• Arjun Sarin 🎉🎊
• Michael Thies (@mhthies)
• Niklas Hauser (@salkinium)
• Sascha Schade (@strongly-typed)

See our Changelog for more details.