Add initial GitLab CI support
Uses currently proof-of-concept openwrt-ci[1] in order to:
* improve the quality of the codebase in various areas
* decrease code review time and help merging contributions faster
* get automagic feedback loop on various platforms and tools
- out of tree build with OpenWrt SDK on following targets:
* ath79-generic
* imx6-generic
* malta-be
* mvebu-cortexa53
- out of tree native build on x86/64 with GCC (versions 8, 9, 10) and Clang 10
- out of tree native x86/64 static code analysis with cppcheck and
scan-build from Clang 10
1. https://gitlab.com/ynezz/openwrt-ci/
Signed-off-by: Petr Štetiar <ynezz@true.cz>