[Feature request] Check if the patches can be applied by git am

[BKPepe]

Hello guys,

I propose to have option in CI/CD to check if the patches can be applied by git am, which will follow what is exactly written here: https://github.com/openwrt/openwrt/blob/df465b6513053c5f14cf26063979f4024ff56ac9/target/linux/generic/PATCHES#L16

I've noticed not only in packages feed (thats the primary use case for me), but also in core repository that there are some patches, wihch does not have proper subject, commit description and Signed-off-by line.

In the past, @feckert in openwrt/openwrt#10264 tried to add missing patch headers, which leads to following issue openwrt/openwrt#10272.

I think in the beginning it is going to be burden, however, with some first potentional issues, it will stay like it is in the current situation as it still happens e.g. openwrt/openwrt@44ef343 , openwrt/openwrt@3c3614c , but overally it will improve the current state.