[readline] Accept Ctrl-U for "delete to start of line"

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/editstring.c

                                unsigned int character ) __nonnull;
 static void delete_character ( struct edit_string *string ) __nonnull;
 static void backspace ( struct edit_string *string ) __nonnull;
+static void kill_sol ( struct edit_string *string ) __nonnull;
 static void kill_eol ( struct edit_string *string ) __nonnull;
 
 /**
 	}
 }
 
+/**
+ * Delete to start of line
+ *
+ * @v string           Editable string
+ */
+static void kill_sol ( struct edit_string *string ) {
+	size_t old_cursor = string->cursor;
+	string->cursor = 0;
+	insert_delete ( string, old_cursor, NULL );
+}
+
 /**
  * Delete to end of line
  *
 		/* Delete character */
 		delete_character ( string );
 		break;
+	case CTRL_U:
+		/* Delete to start of line */
+		kill_sol ( string );
+		break;
 	case CTRL_K:
 		/* Delete to end of line */
 		kill_eol ( string );