diff --git a/Makefile b/Makefile
index 5fa37a4322..21964c16b1 100644
--- a/Makefile
+++ b/Makefile
@@ -36,11 +36,11 @@ ifneq ($(GLUON_BRANCH),)
 endif
 
 ifneq ($(GLUON_FEATURES)$(GLUON_FEATURES_standard)$(GLUON_FEATURES_tiny),)
-  $(error *** Warning: GLUON_FEATURES has been obsolete, please use the image-customization.lua file instead.)
+  $(error *** Warning: GLUON_FEATURES has been obsolete, please use the image-customization.lua file instead)
 endif
 
 ifneq ($(GLUON_SITE_PACKAGES)$(GLUON_SITE_PACKAGES_standard)$(GLUON_SITE_PACKAGES_tiny),)
-  $(error *** Warning: GLUON_SITE_PACKAGES has been obsolete, please use the image-customization.lua file instead.)
+  $(error *** Warning: GLUON_SITE_PACKAGES has been obsolete, please use the image-customization.lua file instead)
 endif
 
 GLUON_AUTOUPDATER_ENABLED ?= 0
diff --git a/docs/user/site.rst b/docs/user/site.rst
index 7f8f544be6..fcd245e4f3 100644
--- a/docs/user/site.rst
+++ b/docs/user/site.rst
@@ -728,6 +728,10 @@ disable_factory()
   Disables factory image generation. Sysupgrade images are still generated and stored in the image
   output directory.
 
+include(path)
+  Includes another image customization file. Relative paths are interpreted relative to the site
+  repository root. Values returned from the included file become the return value of ``include``.
+
 Technically, the image customzation file is evaluated once for each device, allowing
 to make use of regular Lua *if* statements for device-specific configuration as
 can be seen in the example.
diff --git a/scripts/image_customization_lib.lua b/scripts/image_customization_lib.lua
index 3abe9d6943..3a4677aca0 100644
--- a/scripts/image_customization_lib.lua
+++ b/scripts/image_customization_lib.lua
@@ -33,7 +33,7 @@ local function evaluate_device(env, dev)
 	function funcs.broken(broken)
 		assert(
 			type(broken) == 'boolean',
-			'Incorrect use of broken(): has to be a boolean value')
+			'incorrect use of broken(): has to be a boolean value')
 		add_override('broken', broken)
 	end
 
@@ -48,7 +48,7 @@ local function evaluate_device(env, dev)
 	function funcs.device(device_names)
 		assert(
 			type(device_names) == 'table',
-			'Incorrect use of device(): pass a list of device names as argument')
+			'incorrect use of device(): pass a list of device names as argument')
 
 		for _, device_name in ipairs(device_names) do
 			if device_name == dev.image then
@@ -62,7 +62,7 @@ local function evaluate_device(env, dev)
 	function funcs.target(target, subtarget)
 		assert(
 			type(target) == 'string',
-			'Incorrect use of target(): pass a target name as first argument')
+			'incorrect use of target(): pass a target name as first argument')
 
 		if target ~= env.BOARD then
 			return false
@@ -79,6 +79,18 @@ local function evaluate_device(env, dev)
 		return dev.options.class == class
 	end
 
+	function funcs.include(path)
+		if string.sub(path, 1, 1) ~= '/' then
+			assert(
+				string.find(path, '/') == nil,
+				'incorrect use of include(): including files from subdirectories is unsupported')
+			path = env.GLUON_SITEDIR .. '/' .. path
+		end
+		local f = assert(loadfile(path))
+		setfenv(f, funcs)
+		return f()
+	end
+
 	-- Evaluate the feature definition files
 	setfenv(M.customization_file, funcs)
 	M.customization_file()