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>

 # Build script for m4

 CT_M4_VERSION=1.4.13

 do_companion_tools_m4_get() {

     CT_GetFile "m4-${CT_M4_VERSION}" \

                {ftp,http}://ftp.gnu.org/gnu/m4

 }

 do_companion_tools_m4_extract() {

     CT_Extract "m4-${CT_M4_VERSION}"

     CT_Patch "m4" "${CT_M4_VERSION}"

 }

 do_companion_tools_m4_build() {

     CT_DoStep EXTRA "Installing m4"

     mkdir -p "${CT_BUILD_DIR}/build-m4"

     CT_Pushd "${CT_BUILD_DIR}/build-m4"

     CT_DoExecLog CFG \

     "${CT_SRC_DIR}/m4-${CT_M4_VERSION}/configure" \

         --prefix="${CT_BUILDTOOLS_PREFIX_DIR}"

     CT_DoExecLog ALL make

     CT_DoExecLog ALL make install

     CT_Popd

     CT_EndStep

 }