add 'include keyboard_features.mk' into build_keyboard.mk (qmk#8422)

* add 'include keyboard_features.mk' into build_keyboard.mk

keyboard_features.mk is a keyboard-local version of the functions performed by common_features.mk.

* add comment into build_keyboard.mk

* added description of keyboard_features.mk in hardware_keyboard_guidelines.md.

* rename `keyboard_features.mk` to `post_rules.mk`

build_keyboard.mk

@@ -115,6 +115,7 @@ include $(INFO_RULES_MK)
 # Check for keymap.json first, so we can regenerate keymap.c
 include build_json.mk
 
+# Pull in keymap level rules.mk
 ifeq ("$(wildcard $(KEYMAP_PATH))", "")
     # Look through the possible keymap folders until we find a matching keymap.c
     ifneq ("$(wildcard $(MAIN_KEYMAP_PATH_5)/keymap.c)","")
@@ -345,6 +346,7 @@ ifeq ("$(USER_NAME)","")
 endif
 USER_PATH := users/$(USER_NAME)
 
+# Pull in user level rules.mk
 -include $(USER_PATH)/rules.mk
 ifneq ("$(wildcard $(USER_PATH)/config.h)","")
     CONFIG_H += $(USER_PATH)/config.h
@@ -356,6 +358,23 @@ endif
 # Disable features that a keyboard doesn't support
 -include disable_features.mk
 
+# Pull in post_rules.mk files from all our subfolders
+ifneq ("$(wildcard $(KEYBOARD_PATH_1)/post_rules.mk)","")
+    include $(KEYBOARD_PATH_1)/post_rules.mk
+endif
+ifneq ("$(wildcard $(KEYBOARD_PATH_2)/post_rules.mk)","")
+    include $(KEYBOARD_PATH_2)/post_rules.mk
+endif
+ifneq ("$(wildcard $(KEYBOARD_PATH_3)/post_rules.mk)","")
+    include $(KEYBOARD_PATH_3)/post_rules.mk
+endif
+ifneq ("$(wildcard $(KEYBOARD_PATH_4)/post_rules.mk)","")
+    include $(KEYBOARD_PATH_4)/post_rules.mk
+endif
+ifneq ("$(wildcard $(KEYBOARD_PATH_5)/post_rules.mk)","")
+    include $(KEYBOARD_PATH_5)/post_rules.mk
+endif
+
 ifneq ("$(wildcard $(KEYMAP_PATH)/config.h)","")
     CONFIG_H += $(KEYMAP_PATH)/config.h
 endif

docs/hardware_keyboard_guidelines.md

@@ -144,10 +144,38 @@ The `rules.mk` file can also be placed in a sub-folder, and its reading order is
         * `keyboards/top_folder/sub_1/sub_2/sub_3/sub_4/rules.mk`
           * `keyboards/top_folder/keymaps/a_keymap/rules.mk`
           * `users/a_user_folder/rules.mk`
+        * `keyboards/top_folder/sub_1/sub_2/sub_3/sub_4/post_rules.mk`
+      * `keyboards/top_folder/sub_1/sub_2/sub_3/post_rules.mk`
+    * `keyboards/top_folder/sub_1/sub_2/post_rules.mk`
+  * `keyboards/top_folder/sub_1/post_rules.mk`
+* `keyboards/top_folder/post_rules.mk`
 * `common_features.mk`
 
 Many of the settings written in the `rules.mk` file are interpreted by `common_features.mk`, which sets the necessary source files and compiler options.
 
+The `post_rules.mk` file can interpret `features` of a keyboard-level before `common_features.mk`.  For example, when your designed keyboard has the option to implement backlighting or underglow using rgblight.c, writing the following in the `post_rules.mk` makes it easier for the user to configure the `rules.mk`.
+
+* `keyboards/top_folder/keymaps/a_keymap/rules.mk`
+  ```makefile
+  # Please set the following according to the selection of the hardware implementation option.
+  RGBLED_OPTION_TYPE = backlight   ## none, backlight or underglow
+  ```
+* `keyboards/top_folder/post_rules.mk`
+  ```makefile
+  ifeq ($(filter $(strip $(RGBLED_OPTION_TYPE))x, nonex backlightx underglowx x),)
+     $(error unknown RGBLED_OPTION_TYPE value "$(RGBLED_OPTION_TYPE)")
+  endif
+
+  ifeq ($(strip $(RGBLED_OPTION_TYPE)),backlight)
+    RGBLIGHT_ENABLE = yes
+    OPT_DEFS += -DRGBLED_NUM=30
+  endif
+  ifeq ($(strip $(RGBLED_OPTION_TYPE)),underglow)
+    RGBLIGHT_ENABLE = yes
+    OPT_DEFS += -DRGBLED_NUM=6
+  endif
+  ```
+
 ?> See `build_keyboard.mk` and `common_features.mk` for more details.
 
 ### `<keyboard_name.c>`