Add userspace config.h handling to build script (#2640)

* Add userspace 'config.h' file

* Add more robust docs

* Remove config.h code from drashna userspace

* Spelling error

* Include links to Config Options page

* Remove config.h documentation from userspace doc, as it's no longer needed

users/drashna/rules.mk

@@ -2,10 +2,4 @@
 SRC += drashna.c
 EXTRAFLAGS        += -flto
 
-ifneq ("$(wildcard users/$(KEYMAP)/config.h)","")
-    CONFIG_H += users/$(KEYMAP)/config.h
-endif
 
-ifeq ($(strip $(NO_SECRETS)), yes)
-    OPT_DEFS += -DNO_SECRETS
-endif