oeqa/selftest: verify that devtool can use plugins in other layers

In selftest we want to exercise devtool's ability to use plugins from other
layers, so add a basic command to reverse a string to meta-selftest and a test
in oe-selftest to call it.

(From OE-Core rev: 69f5bbc1041e15691af0b4845e2136957a4846a1)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/lib/devtool/test.py

@@ -0,0 +1,11 @@
+import argparse
+
+def selftest_reverse(args, config, basepath, workspace):
+    """Reverse the value passed to verify the plugin is executing."""
+    print args.value[::-1]
+
+def register_commands(subparsers, context):
+    parser_build = subparsers.add_parser('selftest-reverse', help='Reverse value (for selftest)',
+                                         formatter_class=argparse.ArgumentDefaultsHelpFormatter)
+    parser_build.add_argument('value', help='Value to reverse')
+    parser_build.set_defaults(func=selftest_reverse)