base.bbclass: Ensure an empty do_build tasks exists to silence a warning

The message "WARNING: Function do_build doesn't exist" doesn't look professional,
so fix the underlying problem even if this warning is harmless.

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>

meta/classes/base.bbclass

@@ -438,6 +438,9 @@ base_do_package() {
 addtask build after do_populate_sysroot
 do_build = ""
 do_build[func] = "1"
+do_build () {
+	:
+}
 
 python () {
     import exceptions, string