image/testimage: Rework auto image test execution

The TEST_IMAGE interface has never particularly worked and image testing currently
gets configured manually.

This reworks the interface to better serve the needs of its users, replacing it
with TESTIMAGE_AUTO.

This does away with the need for the separate class due to better bitbake APIs for
changing tasks.

TESTIMAGE_AUTO will automatically boot under qemu any image that is built. It
also adds in dependencies so that any SDK for which testing is requested will
automatically be built first.

The code in bitbake.conf was error prone (e.g. testsdk wasn't considered), this
improves it to standardise on IMAGE_CLASSES as the standard configuration mechanism.

(From OE-Core rev: b34d44f3dfea8254826a46701a4fe3769a900434)

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

meta/classes/testsdk.bbclass

@@ -194,3 +194,8 @@ python do_testsdkext() {
 addtask testsdkext
 do_testsdkext[nostamp] = "1"
 
+python () {
+    if oe.types.boolean(d.getVar("TESTIMAGE_AUTO") or "False"):
+        bb.build.addtask("testsdk", None, "do_populate_sdk", d)
+        bb.build.addtask("testsdkext", None, "do_populate_sdk_ext", d)
+}