[libc] Add stdbool.h standard header

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

src/include/curses.h

 #define CURSES_H
 
 #include <stdint.h>
+#include <stdbool.h>
 #include <stdarg.h>
 #include <ipxe/console.h>
 
 #undef  TRUE
 #define TRUE	(1)
 
-typedef int bool;
 typedef uint32_t chtype;
 typedef uint32_t attr_t;
 

src/include/stdbool.h

+#ifndef _STDBOOL_H
+#define _STDBOOL_H
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#define bool _Bool
+#define true 1
+#define false 0
+
+#endif /* _STDBOOL_H */