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.
(From OE-Core rev: 6b9e8b780dcd8d5ffba3df35cfe41674413ee26d)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>