kernel/linux: add alternate download locations

Since kernel.org is dead, and there is no announced or known estimated
time or return to normality, it is impossible to download any kernel at
this time.

Add a known-working mirror.

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

}