sstate.bbclass: Add an extra check for sstate_mirrors

sstate.bbclass: Add an extra check for sstate_mirrors

BB_NO_NETWORK disables any fetching, however if we're using an external
sstate cache, we may want to be able to fetch those objects even if we are
not fetching the upstream sources.  Denote this situation by setting
SSTATE_MIRROR_ALLOW_NETWORK in local.conf.  When it is found, for sstate
cache fetches, mask off BB_NO_NETWORK for the local function.

(From OE-Core rev: ed585cad2e1fdc323c05fa82055a071bcf98d1bc)

Signed-off-by: Joe MacDonald <joe.macdonald@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>