CI: packages: Add github CI job to build all packages

CI: packages: Add github CI job to build all packages

This will build OpenWrt for MIPS malta BE and x86 64 Bit with all
packages and kernel modules activated. It is triggered when something
changes in the build system or when a package definition is changed.
This task probably needs 90 minutes to execute, but I hope that it
will find build problems in pull requests early.

This intentionally does not activate the feeds, because building them
too would take too long. We only build x86/64 and malta/be to save
resources.

I would like to detect build problems when a package is changed. We
often had build breaks when a package version was increased sometime
even in other packages which used it as a dependency.

This is based on the .github/workflows/packages.yml workflow.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>