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 IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple, override only for 64-bit

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

        CT_TARGET_ARCH="mips64${target_endian_el}"

    else

        # The architecture part of the tuple:

        CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

    fi

    # Override CFLAGS for endianness:

    case "${CT_ARCH_BE},${CT_ARCH_LE}" in

        y,) CT_ARCH_ENDIAN_CFLAG="-EB";;

        ,y) CT_ARCH_ENDIAN_CFLAG="-EL";;

    esac

    # Override ABI flags

    CT_ARCH_ABI_CFLAG="-mabi=${CT_ARCH_mips_ABI}"

    CT_ARCH_WITH_ABI="--with-abi=${CT_ARCH_mips_ABI}"

}