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9boot: use BIOS tick counter instead of INT 0x15 WAIT

To implement keyboard input timeout, we used
to poll the keyboard in a loop with a 100ms
sleep in between using INT 0x15 WAIT BIOS call.

But it seems iPXE breaks the BIOS call, making
it just hang.

The BIOS has a 32-bit tick counter at
040:006C that has a period of 54.9254ms.

Using that instead of the WAIT call appears to be
more reliable and makes it work in iPXE.

Implementation note:

Because the counter has a wrap around value
at non-power-of-two, only check if the counter changed
and counting down the millisecond timeout when it did.

--- a/sys/src/boot/pc/fns.h

+++ b/sys/src/boot/pc/fns.h

@@ -10,7 +10,6 @@

 void start(void *sp);

 void cgaputc(int c);

 int kbdgetc(void);

-void usleep(int t);

 void halt(void);

 void jump(void *pc);

--- a/sys/src/boot/pc/l.s

+++ b/sys/src/boot/pc/l.s

@@ -145,17 +145,6 @@

 	ANDL $0xFFFF, AX

 	RET

-TEXT usleep(SB), $0

-	MOVL t+4(SP), AX

-	PUSHL AX

-	CALL rmode16(SB)

-	STI

-	POPR(rDX)

-	POPR(rCX)

-	MOVB $0x86, AH

-	BIOSCALL(0x15)

-	JMP _pret32

-

 #ifdef PXE

 TEXT pxeinit(SB), $0

--- a/sys/src/boot/pc/sub.c

+++ b/sys/src/boot/pc/sub.c

@@ -165,11 +165,18 @@

 static int

 timeout(int ms)

 {

+	long v;

+	volatile long *ticks = (long *) 0x46c;

+

+	v = *ticks;

 	while(ms > 0){

 		if(getc() != 0)

 			return 1;

-		usleep(100000);

-		ms -= 100;

+

+		if (v != *ticks) {

+			ms -= 55;

+			v = *ticks;

+		}

 	}

 	return 0;

 }