scripts/functions: always prefer the mirror

Currently, the mirror can be used either:
- as a fallback in case upstream is unavailable (default behavior)
- as the preferred source for downloads

But the most common use-case seems to provide a truely-LAN mirror
to speed up downloads in big corpos', and/or provide a 'trusted'
source for the tarballs.

So, make the following changes;
- if a mirror is specified, always try that before trying upstream

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...