This allows to use these variables inside build.mk regardless of CWD
and is more clear than a cascade of ".."s

This change also unifies $(submake) and $(modulemk) generation
to reduce duplication.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>