add 'include keyboard_features.mk' into build_keyboard.mk (#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