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>

# Blackfin specific configuration file

## select ARCH_SUPPORTS_32

## select ARCH_DEFAULT_32

## select ARCH_DEFAULT_LE

## select ARCH_SUPPORTS_WITH_ARCH

## select ARCH_SUPPORTS_WITH_CPU

## select ARCH_SUPPORTS_WITH_TUNE

## select ARCH_SUPPORTS_WITH_FPU

##

## help The Blackfin architecture