src/mbr: fix struct mbr packing
src/mbr: fix struct mbr packing

This previous approach caused warnings, altough the layout is exactly
was we need it:
  src/mbr.c:28:10: error: packed attribute causes inefficient alignment
  for ‘disk_signature’ [-Werror=attributes]
Adding a aligned(4) or even aligned(1) attribute does not silence the
warning.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

jluebbe/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc