Add initial GitLab CI support

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 7, 8, 9) and Clang 9
   - out of tree native x86/64 static code analysis with cppcheck and
     scan-build from Clang 9

1. https://gitlab.com/ynezz/openwrt-ci/

Signed-off-by: Petr Štetiar <ynezz@true.cz>