kernel-headers: platform/target patches are not applied

[Martin Schiller]

When preparing the kernel sources to build the kernel headers, currently 
only the patches from the generic folder are applied, but not from the 
actual selected target.

This is basically understandable if one assumes that one wants to use a 
toolchain for different targets with the same architecture.

But if a target has a kernel patch which also changes the uapi, then 
this is not considered in the toolchain/kernel-headers and thus when 
compiling Userspace applications at the moment.

This is currently already the case for the following targets:
* ath79
* bcm27xx
* bcm63xx
* ipq40xx
* lantiq
* realtek

I am currently working on a new target, have now stumbled across this 
problem and would now like to start a discussion here on how best to 
solve this now.

I am looking forward to your ideas.