Current sstate's fetch code doesn't reflect latest fetcher changes, so old
fetch style cause exception and fail silently.
[BUGID #708] got fixed.
Another issue is "import xxx" in python function from sstate.bbclass can only
sit in the head of the function, else have UnboundLocalError: "local variable
XXX referenced before assignment".
Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>