Remove x86/generic target

[Hauke Mehrtens]

Hi,

We would like to remove the 32 bit x86/generic target. The target is 
still working, but we could save some build resources.

See this PR: https://github.com/openwrt/openwrt/pull/14944

Who is still using the x86/generic target? If you are still using it 
what is your use case?

We would keep the x86/64 target which is used on recent x86_64 systems 
and x86_64 cloud systems.

We would also keep the x86/legacy and the x86/geode target. We can 
activate some extra kernel build options if it can not replace 
x86/generic with the current configuration.

The x86/generic target is compiled with Intel Pentium 4 optimizations. 
It will not work on older systems. The x86/legacy and x86/geode target 
are compiled for the Intel Pentium MMX CPU.

Hauke