populate_sdk_ext: Avoid build failures where sstate was used
If sstate was used to populate the build and one of the universal-4.8
or universal-4.9 mirror urls was used, the sstate checks during eSDK
construction could fail as it would zero out the SSTATE_MIRRORs
variable.
Use the same mirrors variable setting as the eSDK would end up using
to perform the checks to avoid this.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
