populate_sdk_ext: Add SSTATE_MIRRORS to config blacklist
SSTATE_MIRRORS used by the builder may not be the same as what the
installer of the sdk will use. Therefore blacklist SSTATE_MIRRORS from
the builder configuration. Note: the actual SSTATE_MIRRORS for the sdk
can be added using sdk_extra_conf.
Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
