scripts: add option to only use the mirror

Currently, if downloads are forbidden, the mirror is still tried for.

Change this way:
- if downlaods forbidden, do not try neither upstream locations nor mirror
- add option to only use the mirror, and avoid upstream locations

Signed-off-by: Austin Morton <austinpmorton@gmail.com>
[yann.morin.1998@free.fr: broaden the if USE_MIRRORto enclode mirror location]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

# Compute sparc-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="${CT_ARCH}64"

    fi

}