own-mirrors.bbclass: allow mirroring of scm fetched packages

own-mirrors.bbclass: allow mirroring of scm fetched packages

this way, it's possible to setup a local webserver (for example
using busybox httpd -p "8081" -h backuped_download_dir) serving
a presiously fetched download directory and to build wihout the
need for an internet access

this can also be used when connected to know to know which packages
are missing from the local mirror's directory (and thus are fetched
from internet as a fallback), it's possible to run the server this way :
busybox httpd -p "8081" -h backuped_download_dir -vv -f | grep -B 1  response:404
to get the name of the missing packages.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Denys Dmytriyenko <denis@denix.org>
Acked-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: Tom Rini <tom_rini@mentor.com>