Support booting from MBR disks

Adds (optional) MBR logic that sets the base sector and sector count used by the redoxfs module dynamically, from the four MBR partitions, rather than including the file system directly and using labels. This is backwards-compatible, since it's only enabled when neither FILESYSTEM nor KERNEL is set, in which case it previously simply wouldn't have worked.

x86_64/bootsector.asm

@@ -86,45 +86,6 @@ load:
     jc error
     ret
 
-  ; store some sectors to disk from a buffer in memory
-  ; buffer has to be below 1MiB
-  ; IN
-  ;   ax: start sector
-  ;   bx: offset of buffer
-  ;   cx: number of sectors (512 Bytes each)
-  ;   dx: segment of buffer
-  ; CLOBBER
-  ;   ax, bx, cx, dx, si
-  ; TODO rewrite to (eventually) move larger parts at once
-  ; if that is done increase buffer_size_sectors in startup-common to that (max 0x80000 - startup_end)
-  store:
-      cmp cx, 127
-      jbe .good_size
-
-      pusha
-      mov cx, 127
-      call store
-      popa
-      add ax, 127
-      add dx, 127 * 512 / 16
-      sub cx, 127
-
-      jmp store
-  .good_size:
-      mov [DAPACK.addr], eax
-      mov [DAPACK.buf], bx
-      mov [DAPACK.count], cx
-      mov [DAPACK.seg], dx
-
-      call print_dapack
-
-      mov dl, [disk]
-      mov si, DAPACK
-      mov ah, 0x43
-      int 0x13
-      jc error
-      ret
-
 print_dapack:
     mov al, 13
     call print_char
@@ -189,6 +150,7 @@ DAPACK:
 .seg:   dw 0 ; in memory page zero
 .addr:  dq 0 ; put the lba to read in this spot
 
-times 510-($-$$) db 0
+times 446-($-$$) db 0
+partitions: times 4 * 16 db 0
 db 0x55
 db 0xaa