sstate.bbclass: fixes for SSTATE_SIG_KEY when use SSTATE_MIRRORS
Fixed:
WARNING: Failed to fetch URL file://Ubuntu-12.04/39/sstate:m4-native:x86_64-linux:1.4.17:r0:x86_64:3:39ecb836d5fc7802ce6f45f33f1cae36_populate_sysroot.tgz.sig, attempting MIRRORS if available
ERROR: Fetcher failure: Unable to find file file://Ubuntu-12.04/39/sstate:m4-native:x86_64-linux:1.4.17:r0:x86_64:3:39ecb836d5fc7802ce6f45f33f1cae36_populate_sysroot.tgz.sig anywhere. The paths that were searched were:
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>