Makefile: use abolute paths in generated variables

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>

Makefile

@@ -25,21 +25,13 @@ all: $(submake) $(modulemk)
 	$(call only-if, $(mgmt), cd mgmt && ./gradlew --daemon :web:jar build)
 	$(MAKE) -r -C $(dir $(submake)) $@
 
-$(submake): Makefile
+$(submake) $(modulemk): Makefile
 	mkdir -p $(dir $@)
 	echo 'mode = $(mode)' > $@
-	echo 'src = ../..' >> $@
+	echo 'src = $(abspath .)' >> $@
 	echo 'out = $(abspath $(out))' >> $@
-	echo 'VPATH = ../..' >> $@
-	echo 'include ../../build.mk' >> $@
-
-$(modulemk): Makefile
-	mkdir -p $(dir $@)
-	echo 'mode = $(mode)' > $@
-	echo 'src = ../../../..' >> $@
-	echo 'out = $(abspath $(out))' >> $@
-	echo 'VPATH = ../../../../' >> $@
-	echo 'include ../../../../build.mk' >> $@
+	echo 'VPATH = $(abspath .)' >> $@
+	echo 'include $(abspath build.mk)' >> $@
 
 clean:
 	$(call quiet, rm -rf build/$(mode), CLEAN)