where backup links are available, use those as main instead.
this is because of the new XBMK_CACHE_MIRROR variable, which
makes --mirror be used
when performed on review.coreboot.org, this also pulls down
all changes from gerrit code review; the github backups for
example only contain the official branches, but gerrit creates
a new ref per merge request.
a user can still run ./mk -F to force pulling all repos,
including the coreboot.org ones, but use of -f will skip the
coreboot.org ones if the backup links worked and contain the
local commit needed, by a given project used in xbmk.
this patch won't change any real-world behaviour for xbmk
users, but it is done as a courtesy to the coreboot project,
in that it largely avoids a sudden surge in coreboot.org's
traffic if lots of users start doing XBMK_CACHE_MIRROR=y
if XBMK_CACHE_MIRROR is not set, or set to anything other
than y, a regular clone is performed, saving cached sources
to cache/clone/ - otherwise, cache/mirror/ is used.
Signed-off-by: Leah Rowe <leah@libreboot.org>