Convert CONFIG_KEYBOARD to Kconfig

This converts the following to Kconfig:
   CONFIG_KEYBOARD

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/input/Kconfig b/drivers/input/Kconfig
index a17e55e..0b753f3 100644
--- a/drivers/input/Kconfig
+++ b/drivers/input/Kconfig
@@ -38,6 +38,15 @@
 	  includes methods to start/stop the device, check for available
 	  input and update LEDs if the keyboard has them.
 
+config KEYBOARD
+	bool "Enable legacy keyboard support (deprecated)"
+	help
+	  Enable this to enable a custom keyboard support.
+	  This simply calls drv_keyboard_init() which must be
+	  defined in your board-specific files. This option is deprecated
+	  and is only used by novena. For new boards, use driver model
+	  instead.
+
 config CROS_EC_KEYB
 	bool "Enable Chrome OS EC keyboard support"
 	depends on INPUT