complibs: introduce a two-stage select

Directly select-ing the required companion libraries means we can not
disable them. That's OK for now, as we systematically build them when
they are required.

But with distros coming up-to-speed, we will need to disable the build
later-on.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Blackfin specific configuration file

config ARCH_blackfin

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_DEFAULT_LE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    select ARCH_SUPPORT_FPU

     help

      The Blackfin architecture