build: Rebuild when Portage config changes

The `host-tools` and `build` targets will now be rebuilt automatically
when the respective Portage configuration changes.

Makefile

@@ -17,6 +17,7 @@ prepare: $(O)/.prepared
 $(O)/.host-tools: \
 		build-host-tools.sh \
 		host-tools.packages \
+		$(shell find portage/config/host -type f) \
 		$(O)/.prepared
 	./build-host-tools.sh
 	touch $(O)/.host-tools
@@ -27,6 +28,7 @@ $(O)/.built: \
 		build.sh \
 		build.packages \
 		install.packages \
+		$(shell find portage/config/target -type f) \
 		$(O)/.host-tools \
 		$(O)/.prepared
 	./build.sh