core: msg: introduce msg_try_send
core: adapt to msg_try_send
Merge pull request #1847 from kaspar030/msg_try_send

core: introduce msg_try_send
sys: lib: ringbuffer: introduce ringbuffer_get_free

This function returns the number of bytes available in the ringbuffer.
Merge pull request #1650 from kaspar030/ringbuffer_get_free

sys: lib: ringbuffer: introduce ringbuffer_get_free
dist: add Dockerfile for docker builder container
Merge pull request #1637 from kaspar030/docker

dist: add Dockerfile for docker builder container
cpu/cc2538: added linkerscript for cc2538sf53
boards: added support for OpenMote
cpu/cc2538: cleaned up IRQ vector definition
cpu/cc2538: centralized all cpu specific includes

- added all CPU specific includes to cpu-conf.h
- removed now not needed includes from cpu .c files
cpu/cc2538: remodeled GPIO driver
cpu/cc2538: adjusted GPIO definitions