Verify header checksum

Calculate the checksum for the boot header and compare it with the
checksum contained in the header.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Check 'Width Detection' field

The width detection field must match a certain value. Check that it is
set correctly.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>