sstate: Make SSTATE_SCAN_CMD vars configurable using weak defaults.

For this we move them out of the python section so they can be
overridden on a per-recipe basis.

The motivation for this change is that not all tool chains need the
path modifications provided by the command, and these will provide
alternative or empty commands. The Go compiler is such an example.

(From OE-Core rev: 7d2a2160336413736dd4640f5b84ba4d74bb00f5)

Signed-off-by: Kristian Amlie <kristian.amlie@mender.io>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/native.bbclass

@@ -115,6 +115,10 @@ MACHINEOVERRIDES = ""
 
 PATH_prepend = "${COREBASE}/scripts/native-intercept:"
 
+# This class encodes staging paths into its scripts data so can only be
+# reused if we manipulate the paths.
+SSTATE_SCAN_CMD ?= "${SSTATE_SCAN_CMD_NATIVE}"
+
 python native_virtclass_handler () {
     classextend = e.data.getVar('BBCLASSEXTEND') or ""
     if "native" not in classextend: