[serial] Don't enable serial console without serial support

serial_console_init() would enable serial console support without
knowing if the serial driver succeeded or not.  As a result, the
serial console would interfere with a normal keyboard on a system
lacking serial support.

Reported-by: Jan ONDREJ (SAL) <ondrejj(at)salstar.sk>
Signed-off-by: Shao Miller <sha0.miller@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/serial.c

 #define uart_writeb(val,addr) outb((val),(addr))
 #endif
 
+/* Boolean for the state of serial driver initialization */
+int serial_initialized = 0;
+
 /*
  * void serial_putc(int ch);
  *	Write character `ch' to port UART_BASE.
 	/* Set clear to send, so flow control works... */
 	uart_writeb((1<<1), UART_BASE + UART_MCR);
 
-
 	/* Flush the input buffer. */
 	do {
 		/* rx buffer reg
 		/* line status reg */
 		status = uart_readb(UART_BASE + UART_LSR);
 	} while(status & UART_LSR_DR);
+
+	/* Note that serial support has been initialized */
+	serial_initialized = 1;
  out:
 	return;
 }

src/core/serial_console.c

 struct console_driver serial_console __console_driver;
 
 static void serial_console_init ( void ) {
-	/* Serial driver initialization should already be done,
-	 * time to enable the serial console. */
-	serial_console.disabled = 0;
+	/*
+	 * Check if serial driver initialization is done.
+	 * If so, it's time to enable the serial console.
+	 */
+	if ( serial_initialized )
+		serial_console.disabled = 0;
 }
 
 struct console_driver serial_console __console_driver = {

src/include/ipxe/serial.h

 extern void serial_putc ( int ch );
 extern int serial_getc ( void );
 extern int serial_ischar ( void );
+extern int serial_initialized;
 
 #endif /* _IPXE_SERIAL_H */