Fix generated-sources-dir-for for COMMON

When the 4th argument specified is non-empty then it attempts to
use either HOST_OUT_GEN_COMMON or TARGET_OUT_GEN_COMMON
depending on whether the 3rd argument is non-empty or not
respectively. Unfortunately, those two variables do not exist,
the correct names for those variables is HOST_OUT_COMMON_GEN and
TARGET_OUT_COMMON_GET.

Change-Id: I66edb02824c06e0f504ebe04ff80ddbd77a16c95