Reduce warnings:

- Add prototype for boot2 main()
 - Don't make assignment within if statement, split it into two.

No functional or binary change -- verified with sha256(1).

MFC after:	2 weeks

sys/boot/i386/boot2/boot2.c

@@ -150,6 +150,7 @@ static int comspeed = SIOSPD;
 static uint8_t ioctrl = IO_KEYBOARD;
 #endif
 
+int main(void);
 void exit(int);
 static void load(void);
 static int parse(void);
@@ -481,7 +482,8 @@ parse()
 			     ? DRV_HARD : 0) + drv;
 		dsk_meta = 0;
 	    }
-	    if (k = ep - arg) {
+	    k = ep - arg;
+	    if (k > 0) {
 		if (k >= sizeof(knamebuf))
 		    return -1;
 		memcpy(knamebuf, arg, k + 1);