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/testimage.bbclass

@@ -10,6 +10,11 @@
 # - first add IMAGE_CLASSES += "testimage" in local.conf
 # - build a qemu core-image-sato
 # - then bitbake core-image-sato -c testimage. That will run a standard suite of tests.
+#
+# The tests can be run automatically each time an image is built if you set
+# TESTIMAGE_AUTO = "1"
+
+TESTIMAGE_AUTO ??= "0"
 
 # You can set (or append to) TEST_SUITES in local.conf to select the tests
 # which you want to run for your target.
@@ -382,4 +387,7 @@ def package_extraction(d, test_suites):
 
 testimage_main[vardepsexclude] += "BB_ORIGENV DATETIME"
 
-inherit testsdk
+python () {
+    if oe.types.boolean(d.getVar("TESTIMAGE_AUTO") or "False"):
+        bb.build.addtask("testimage", "do_build", "do_image_complete", d)
+}