build: Add cppflags, cflags, cxxflags to groups

Propagate the group defined cppflags, cflags, and cxxflags from parent groups
to child items through the build item context.

Update #4670.

spec/build/bsps/lm32/lm32_evr/grp.yml

@@ -1,7 +1,10 @@
 SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
 build-type: group
+cflags: []
 copyrights:
 - Copyright (C) 2020 embedded brains GmbH (http://www.embedded-brains.de)
+cppflags: []
+cxxflags: []
 enabled-by: true
 includes: []
 install: []