build: add ci-formal-checks and ci

This does some basic formal testing, just like done for packages.git.
Developers can test their commits locally by running `make
ci-formal-checks`. If working on another branch than `master`, add
`BRANCH=<branch>`. Later compiling of device tree files, checking kmods
or other more specific tests could be done. However this is a first step
to CI.

Signed-off-by: Paul Spooren <mail@aparcar.org>
1 job for ci in 2 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #358
formal

00:02:26

 
Name Stage Failure
failed
formal Test
WARNING: Makefile 'package/boot/kexec-tools/Makefile' has a dependency on 'liblzma', which does not exist
WARNING: Makefile 'package/network/services/lldpd/Makefile' has a dependency on 'libnetsnmp', which does not exist
WARNING: Makefile 'package/network/utils/nftables/Makefile' has a dependency on 'jansson', which does not exist
Error opening terminal: unknown.
make[2]: Leaving directory '/builds/openwrt/openwrt'
make[2]: *** [/builds/openwrt/openwrt/include/toplevel.mk:137: menuconfig] Error 1
make[1]: *** [/builds/openwrt/openwrt/include/toplevel.mk:97: .config] Error 2
make: *** [/builds/openwrt/openwrt/include/toplevel.mk:216: ci-formal-checks] Error 2
ERROR: Job failed: exit code 1