Convert CONFIG_SYS_STDIO_DEREGISTER to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_STDIO_DEREGISTER

This option should never be enabled in SPL, so use
CONFIG_IS_ENABLED(SYS_STDIO_DEREGISTER) when checking the option.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Re-sync]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/Kconfig b/common/Kconfig
index ff2b1a4..dbe5bb6 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -335,6 +335,15 @@
 	  Enable this option to supress this output. It can be obtained by
 	  calling stdio_print_current_devices() from board code.
 
+config SYS_STDIO_DEREGISTER
+	bool "Allow deregistering stdio devices"
+	default y if USB_KEYBOARD
+	help
+	  Generally there is no need to deregister stdio devices since they
+	  are never deactivated. But if a stdio device is used which can be
+	  removed (for example a USB keyboard) then this option can be
+	  enabled to ensure this is handled correctly.
+
 endmenu
 
 config SYS_NO_FLASH