[syslinux] SDI support works correctly (sdi.c32)

[Sergii Kolisnyk]

Hi, Peter and Remi,

thank you a lot!

We still have a little political issue, about checking the header checksum:
http://skolk.livejournal.com/886.html
http://syslinux.zytor.com/archives/2008-April/009878.html

Because it covers only first 512 bytes, and there are no other checksums,
it's not usable for error detection.

But the checksum can be used in addition to $SDI magic to verify format.

The political question is do we need to check it.

Best regards,
Sergii Kolisnyk