mbr: set up first 7 bytes of kernel PSP

boot/mbr.asm

@@ -6,10 +6,12 @@
 
 init:	cli
 	mov ax, seg
-	mov ss, ax
 	mov ds, ax
 	mov es, ax
+	mov ss, ax
 	xor sp, sp
+	mov ax, sp
+	push ax
 
 .retry:	mov ax, 0x0208
 	mov cx, 2
@@ -19,6 +21,18 @@ init:	cli
 	int 0x13
 	jc .retry
 
+	xor di, di
+	mov ax, 0x18CD
+	stosw
+	int 0x12
+	mov cl, 6
+	shl ax, cl
+	stosw
+	mov al, 2
+	stosb
+	mov ax, 0x18CD
+	stosw
+
 	jmp seg:0x100
 
 	times (0x1BE - ($-$$)) db 0