configure: add possibility to set arbitrary variable in check_for

If check_for is able to find the required prog/inc/lib, allow it to
set an arbitrary variable to 'y'. This variable is then pushed down
to the kconfig definition.

For example:
has_or_abort prog=foobar kconfig=has_foobar

If foobar is available, it yields a kconfig variable defaulting to y:
config CONFIGURE_has_foobar
bool
default y

If foobar is missing, it yields a kconfig variable defaulting to n:
config CONFIGURE_has_foobar
bool

Thus it is possible to depends on that variabel to show/hide options:
config SOME_FEATURE
bool
prompt "Some feature"
depends on CONFIGURE_has_foobar

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

 # Compute x86-specific values

 # This one really needs a little love! :-(

 CT_DoArchTupleValues() {

     # Override the architecture part of the tuple:

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

         CT_TARGET_ARCH=x86_64

     else

         arch="${CT_ARCH_ARCH}"

         [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

         case "${arch}" in

             "")                           CT_TARGET_ARCH=i386;;

             i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

             winchip*)                     CT_TARGET_ARCH=i486;;

             pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

             pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

             *)                            CT_TARGET_ARCH=i586;;

         esac

     fi

 }