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>

source "config/configure.in"

source "config/backend.in"

source "config/global.in"

source "config/target.in"

source "config/toolchain.in"

source "config/kernel.in"

source "config/binutils.in"

source "config/cc.in"

source "config/libc.in"

source "config/debug.in"

source "config/companion_libs.in"

source "config/companion_tools.in"

source "config/test_suite.in"