mcextend: Add helper class useful for multiconfig

This class allows users to easily create multiconfig BBCLASSEXTEND recipes
and is generally useful. It will be used by new tests so add as a general
feature.

(From OE-Core rev: 9ad19eb12c24d1ddc967215af0ebafd2cd2cb665)

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

meta/classes/mcextend.bbclass

@@ -0,0 +1,16 @@
+python mcextend_virtclass_handler () {
+    cls = e.data.getVar("BBEXTENDCURR")
+    variant = e.data.getVar("BBEXTENDVARIANT")
+    if cls != "mcextend" or not variant:
+        return
+
+    override = ":virtclass-mcextend-" + variant
+
+    e.data.setVar("PN", e.data.getVar("PN", False) + "-" + variant)
+    e.data.setVar("MCNAME", variant)
+    e.data.setVar("OVERRIDES", e.data.getVar("OVERRIDES", False) + override)
+}
+
+addhandler mcextend_virtclass_handler
+mcextend_virtclass_handler[eventmask] = "bb.event.RecipePreFinalise"
+