image: Ensure we don't expand TMPDIR in image commands

Similarly to DATETIME, don't expand TMPDIR in image commands.
This ensures some of the stamp comparisons we make in the
QA tests work correctly.

(From OE-Core rev: a8c377beadb85b0ff503ec8ddd1a2cd05e363c19)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image.bbclass

@@ -369,7 +369,9 @@ python () {
         # Delete DATETIME so we don't expand any references to it now
         # This means the task's hash can be stable rather than having hardcoded
         # date/time values. It will get expanded at execution time.
+        # Similarly TMPDIR since otherwise we see QA stamp comparision problems
         localdata.delVar('DATETIME')
+        localdata.delVar('TMPDIR')
 
         image_cmd = localdata.getVar("IMAGE_CMD", True)
         vardeps.add('IMAGE_CMD_' + realt)