Move external plugin loading at the end of erlang.mk

This allows all plugins to append to existing variables
and makes it easier to augment the functionality of the
core plugins.

build.config

@@ -40,5 +40,8 @@ plugins/xref
 plugins/cover
 plugins/sfx
 
+# External plugins.
+core/plugins
+
 # Core modules which can use variables from plugins.
 core/deps-tools

core/deps.mk

@@ -591,18 +591,3 @@ ERLANG_MK_RECURSIVE_DOC_DEPS_LIST = $(ERLANG_MK_TMP)/recursive-doc-deps-list.log
 ERLANG_MK_RECURSIVE_REL_DEPS_LIST = $(ERLANG_MK_TMP)/recursive-rel-deps-list.log
 ERLANG_MK_RECURSIVE_TEST_DEPS_LIST = $(ERLANG_MK_TMP)/recursive-test-deps-list.log
 ERLANG_MK_RECURSIVE_SHELL_DEPS_LIST = $(ERLANG_MK_TMP)/recursive-shell-deps-list.log
-
-# External plugins.
-
-DEP_PLUGINS ?=
-
-define core_dep_plugin
--include $(DEPS_DIR)/$(1)
-
-$(DEPS_DIR)/$(1): $(DEPS_DIR)/$(2) ;
-endef
-
-$(foreach p,$(DEP_PLUGINS),\
-	$(eval $(if $(findstring /,$p),\
-		$(call core_dep_plugin,$p,$(firstword $(subst /, ,$p))),\
-		$(call core_dep_plugin,$p/plugins.mk,$p))))

core/plugins.mk

@@ -0,0 +1,17 @@
+# Copyright (c) 2013-2017, Loïc Hoguin <essen@ninenines.eu>
+# This file is part of erlang.mk and subject to the terms of the ISC License.
+
+# External plugins.
+
+DEP_PLUGINS ?=
+
+define core_dep_plugin
+-include $(DEPS_DIR)/$(1)
+
+$(DEPS_DIR)/$(1): $(DEPS_DIR)/$(2) ;
+endef
+
+$(foreach p,$(DEP_PLUGINS),\
+	$(eval $(if $(findstring /,$p),\
+		$(call core_dep_plugin,$p,$(firstword $(subst /, ,$p))),\
+		$(call core_dep_plugin,$p/plugins.mk,$p))))