build: selecting an invalid package is silently accepted

[SvenRoederer]

The current buildsystem uses /config to define the packages that should be build.
Adding an invalid package (typo, package got renamed, package got removed) will silently accepted and as it's just ignored by OpenWrts make defconfig. So detecting such problems is not straight forward.

Adding a check, that the listed packages are enabled in the final .config of OpenWrt would be great.