config: add support for a second part of the generated choices

Some components have configuration options that can depend on
generic options, so they should go below those.

uClibc for example:
- has its own options (wchar...)
- uses the generic options (threads...)
- if linuxthreads chosen, offers two impls

So we need to be able to split the components options in 2,
one part that is above the generic options, and one part that
ends up below the generic options.

 # MIPS specific config options

 config ARCH_mips

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64 if EXPERIMENTAL

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_ABI

     select ARCH_SUPPORT_TUNE

     help

       The MIPS architecture, as defined by:

         http://www.mips.com/