Make pc86 echo console output to the serial port, so qemu can pipe it to

stdout.

plat/pc86/README

@@ -23,6 +23,9 @@ are stubs required to make the demo apps link. File descriptors 0, 1 and 2
 represent the console. All reads block. There's enough TTY emulation to allow
 \n conversion and local echo (but it can't be turned off).
 
+Console output is echoed to the serial port (without any setup). This is used
+by qemu for running tests.
+
 
 Example command line
 ====================

plat/pc86/libsys/_sys_rawwrite.s

@@ -21,9 +21,18 @@ __sys_rawwrite:
 	push bp
 	mov bp, sp
 
+	! Write to the BIOS console.
+
 	movb al, 4(bp)
 	movb ah, 0x0E
 	mov bx, 0x0007
 	int 0x10
+
+	! Also write to the serial port (used by the test suite).
+
+	movb ah, 0x01
+	xor dx, dx
+	int 0x14
+
 	jmp .cret