scripts/oe-find-native-sysroot: fix a incomplete regexp

A "=" is missing in the regexp, which leads it match multiple results
if STAGING_DIR_NATIVE has overrides, for instance:
...
STAGING_DIR_NATIVE="xxxx"
STAGING_DIR_NATIVE_qemux86="yyyy"
...

it will match both of them without "=".

(From OE-Core rev: fb9cf84b1d537e2b8c26cd78f6ac27c55a7e79fc)

Signed-off-by: Ming Liu <peter.x.liu@external.atlascopco.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-find-native-sysroot

@@ -50,7 +50,7 @@ BITBAKE_E=""
 set_oe_native_sysroot(){
     echo "Running bitbake -e $1"
     BITBAKE_E="`bitbake -e $1`"
-    OECORE_NATIVE_SYSROOT=`echo "$BITBAKE_E" | grep ^STAGING_DIR_NATIVE | cut -d '"' -f2`
+    OECORE_NATIVE_SYSROOT=`echo "$BITBAKE_E" | grep ^STAGING_DIR_NATIVE= | cut -d '"' -f2`
 
     if [ "x$OECORE_NATIVE_SYSROOT" = "x" ]; then
         # This indicates that there was an error running bitbake -e that