stand: Reduce limit to 500k for x86 loader

The largest loader that works for PXE boot is about 500k. PXE needs low
memory for packets and other driver state, so the largest safe size for
the loader is about 500k. Reduce the size from 560k to 500k so we don't
accidentally break PXE in the future.

Add a comment for people with special needs. If you control the
hardware, it can be safe to have boot loaders as large as 580k or 600k
in some cases. Since the BIOS loader is becoming more and more of a
legacy item, the build variable LOADERSIZE isn't documented. This change
doesn't change that: there's been little demand for this documentation
and in general, users shouldn't change it lightly.

PR: 257018
Sponsored by: Netflix