drivers/Kconfig: Add an option, CONFIG_DRVR_MKRD, to control whether or not the mkrd() implementation is build. Otherwise, if mkrd() is built unconditionally, it will be drawn into every build whether it is used or not and will increase the build size.

drivers/Makefile

@@ -90,7 +90,10 @@ endif
 CSRCS += dev_null.c dev_zero.c
 
 ifneq ($(CONFIG_DISABLE_MOUNTPOINT),y)
-  CSRCS += ramdisk.c mkrd.c
+  CSRCS += ramdisk.c
+ifeq ($(CONFIG_DRVR_MKRD),y)
+  CSRCS += mkrd.c
+endif
 ifeq ($(CONFIG_DRVR_WRITEBUFFER),y)
   CSRCS += rwbuffer.c
 else