Kconfig updates for v5.8
- allow only 'config', 'comment', 'if' statements inside 'choice' since
the other statements are not sensible inside 'choice' and should be
grammatical error
- support LMC_KEEP env variable for 'make local{yes,mod}config' to
preserve some CONFIG options
- deprecate 'make kvmconfig' and 'make xenconfig' in favor of
'make kvm_guest.config' and 'make xen.config'
- code cleanups