kernel - Fix buffer overflow in legacy (non EFI) boot loader

kernel - Fix buffer overflow in legacy (non EFI) boot loader

* Fix a buffer overflow primarily in bd_read().  The code contains
  a work-around for segment boundary crossings that very old BIOSes
  would sometimes blow-up on, but was improperly calculating the
  bounce-buffer I/O limit to be the full bounce-buffer size
  instead of 1/2 the bounce buffer size.

  Thus if the bounce buffer's base address happened to be certain
  values, large I/O requests would overflow the buffer.

* Make sure the bounce-buffer is 16-byte aligned, again for old
  BIOSes.

Reported-by: zrj
Debugging-by: dillon, zrj, Naabed-