[menu] Adapt user interface to fit display size

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

src/hci/tui/menu_ui.c

 #define CPAIR_SEPARATOR	3
 
 /* Screen layout */
-#define TITLE_ROW	1
-#define MENU_ROW	3
-#define MENU_COL	1
-#define MENU_ROWS	18
-#define MENU_COLS	78
-#define MENU_PAD	2
+#define TITLE_ROW	1U
+#define MENU_ROW	3U
+#define MENU_COL	1U
+#define MENU_ROWS	( LINES - 2U - MENU_ROW )
+#define MENU_COLS	( COLS - 2U )
+#define MENU_PAD	2U
 
 /** A menu user interface */
 struct menu_ui {
 	/** Menu */
 	struct menu *menu;
 	/** Number of menu items */
-	int count;
+	unsigned int count;
 	/** Currently selected item */
 	int selected;
 	/** First visible item */
 			if ( ui->selected < 0 ) {
 				ui->selected = 0;
 				move = +1;
-			} else if ( ui->selected >= ui->count ) {
+			} else if ( ui->selected >= ( int ) ui->count ) {
 				ui->selected = ( ui->count - 1 );
 				move = -1;
 			}