Makefile: Use CURDIR instead of PWD

Fixes setup of BASE_DIR git repositories when used like: make -C
system-images update

Related: SYS#3967

Makefile

@@ -112,10 +112,10 @@ build.%: | git/poky
 
 	# Append entries to conf/local.conf. Common first, machine second... filter
 	$(Q)$(foreach file,$(CFG_FILES), \
-		cat $(file) | sed s,BASE_DIR,$(PWD), >> $@/conf/local.conf;)
+		cat $(file) | sed s,BASE_DIR,${CURDIR}, >> $@/conf/local.conf;)
 	@echo "require conf/distro/include/sysmocom-defaults.conf" >> $@/conf/local.conf
 
-	$(Q)cat cfg/bblayers.conf | sed  s,BASE_DIR,$(PWD), > $@/conf/bblayers.conf
+	$(Q)cat cfg/bblayers.conf | sed  s,BASE_DIR,${CURDIR}, > $@/conf/bblayers.conf
 
 
 # generic git pull --rebase rule. Let's assume this is matching poky-update