functions: add new helpers that create a dir and cd/pushd into it

A lot of places are currently doing:
mkdir -p foo/bar
cd foo/bar

Or even:
mkdir -p foo/bar
pushd foo/bar
[...]
popd

Provide both wrapper to ease doing this.

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

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}