Add placeholder variable for meta-clang addition

Add placeholder variable for meta-clang addition

meta-clang is a dependency of multiple layers. Thus to avoid double inclusions,
add this placeholder and indirection.
To add the clang layer, you have to define AGL_META_CLANG instead of changing
BBLAYERS directly.

Bug-AGL: SPEC-3852
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Change-Id: I32ae8b5027531c3fc7a38c15ff3c0269c85ed9b8
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26984
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account