menu: Start bootmenu_key values at 0x100

At present the backspace key (Ctrl-B) has the same value as BKEY_UP,
which means the keys are ambiguous. This causes problems when trying
to edit a textline. The same problem applies to other keys used in
cread_line_process_ch()

Start the bootmenu_key enum values at 0x100 to avoid this collision.
Keep BKEY_NONE as 0, since it is a sentinel value.

Co-developed-by: Claude <claude@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

include/menu.h

@@ -45,10 +45,16 @@ struct bootmenu_data {
 	struct bootmenu_entry *first;	/* first menu entry */
 };
 
-/** enum bootmenu_key - keys that can be returned by the bootmenu */
+/**
+ * enum bootmenu_key - keys that can be returned by the bootmenu
+ *
+ * These values start at 0x100 to avoid colliding with ASCII control characters
+ * (0x01-0x1f) which are used for editing operations in textlines. BKEY_NONE is
+ * kept at 0 as a sentinel value.
+ */
 enum bootmenu_key {
 	BKEY_NONE = 0,
-	BKEY_UP,
+	BKEY_UP = 0x100,
 	BKEY_DOWN,
 	BKEY_SELECT,
 	BKEY_QUIT,