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.

 # AVR32 specific configuration file

 config ARCH_avr32

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     select ARCH_SUPPORT_FPU

      help

       The AVR32 architecture, as defined by:

         http://www.atmel.com/products/avr32