eficonfig: CTRL+S to save the boot order

The change boot order menu in eficonfig can have at most INT_MAX lines
and it is troublesome to scroll down to the "Save" entry.

This commit assigns CTRL+S to save the boot order.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

include/menu.h

@@ -53,6 +53,7 @@ enum bootmenu_key {
 	BKEY_PLUS,
 	BKEY_MINUS,
 	BKEY_SPACE,
+	BKEY_SAVE,
 
 	BKEY_COUNT,
 };