include/target.mk: default to CPU_TYPE for the package architecture

Use the more specific CPU_TYPE designation as default package architecture
and only fall back to ARCH if no specific CPU_TYPE is set.

This means that e.g. ar71xx packages will use "74kc" as architecture,
not "mips" which is a precondition for sharing packages between targets.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>