The local.conf file can get overritten and changed by the SDK so (ab)use
auto.conf instead to ensure our configuration tweaks get applied.
Also ensure we append to any other configured SSTATE_MIRROR variable
that already exists.
(From OE-Core rev: 444a71b1c2d8f86fc4c35c8ff8e92bcbbe646b75)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>