kernel/linux: use generic custom infrastructure

Config options remain the same as before, just generalised to be used by other
components also.

Signed-off-by: "David Holsgrove" <david.holsgrove@xilinx.com>
[yann.morin.1998@free.fr: fix indentation, fix comment]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <50674fe47431174aab80.1349931193@localhost.localdomain>
PatchWork-Id: 190786

 # Compute sparc-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="${CT_ARCH}64"

     fi

 }