classes/externalsrc: set do_compile as nostamp

Most of the time what you want when using this class is for do_compile
to execute more than just once - every time the source changes would be
ideal, but that's a little tricky to accomplish. Thus, set do_compile as
nostamp to get something close.  Note that in order to be effective this
also requires the change to bitbake that causes nostamp task signatures
to change on each execution.

(From OE-Core rev: e734e12fd1478cdacdadf4e0fae5247cb70b9fb1)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/externalsrc.bbclass

@@ -49,5 +49,8 @@ python () {
 
         for task in d.getVar("SRCTREECOVEREDTASKS", True).split():
             bb.build.deltask(task, d)
+
+        # Ensure compilation happens every time
+        d.setVarFlag('do_compile', 'nostamp', '1')
 }