[syslinux] Read disk not working in 3.35+

[H. Peter Anvin]

Well, there is a bug there anyway... I forgot that touching BP means the 
P_* macros no longer work.  This is a particularly nasty property of 
16-bit code: one can't use [sp+XXX] but have to do [bp+XXX], which means 
using two registers to do the job of one.

I have checked in a fix in git.  I don't know when I'll have a new 
release out, though; I'd like to get 3.40 out so it's probably going to 
be folded into that.

	-hpa