of: make CONFIG_OF user selectable

With the addition of overlays, it is now plausible to use DT on any arch
and without an arch using it at boot time. It is also desirable to
expand the compile coverage of the DT code. Make CONFIG_OF user
selectable by converting the menu to menuconfig.
Signed-off-by: NRob Herring <robh@kernel.org>
Acked-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Acked-by: NPantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: NGrant Likely <grant.likely@lianro.org>

drivers/of/Kconfig

 config DTC
 	bool
 
-config OF
-	bool
+menuconfig OF
+	bool "Device Tree and Open Firmware support"
+	help
+	  This option enables the device tree infrastructure.
+	  It is automatically selected by platforms that need it or can
+	  be enabled manually for unittests, overlays or
+	  compile-coverage.
 
-menu "Device Tree and Open Firmware support"
-	depends on OF
+if OF
 
 config OF_UNITTEST
 	bool "Device Tree runtime unit tests"
@@ -97,4 +101,4 @@ config OF_OVERLAY
 	  While this option is selected automatically when needed, you can
 	  enable it manually to improve device tree unit test coverage.
 
-endmenu # OF
+endif # OF