This way of operation is more robust than the "AI" used
before.
Add flags to mbr accessible from make.conf as BOOT_MBR_FLAGS.
Only one flag is defined now, "allow using packet mode", which
is 0x80 in accord with the rest of i386 boot code. The "packet"
flag is on by default.
PR: i386/70241
Submitted by: Valentin Nechayev <netch <@> netch.kiev.ua> (inital version)
Discussed with: jhb (by Valentin Nechayev)
Tested on: bochs (with EDD turned on or off by patching the BIOS), PCs
e8321616db