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>

 # Overall toolchain configuration: paths, jobs, etc...

 # Allow unconditional usage of tristates

 config MODULES

     bool

     default y

 menu "Paths and misc options"

 source "config/global/ct-behave.in"

 source "config/global/paths.in"

 source "config/global/download.in"

 source "config/global/extract.in"

 source "config/global/build-behave.in"

 source "config/global/logging.in"

 endmenu