Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

$Id$ -> $FreeBSD$

Drop some non-existent commented-out targets.

Initial integration of the i386 bootloader and BTX.

- Discard large amounts of BIOS-related code in favour of the more compact
BTX vm86 interface.
- Build the loader module as ELF, although the resulting object is a.out,
make gensetdefs 32/64-bit sensitive and use a single copy of it.
- Throw away installboot, as it's no longer required.
- Use direct bcopy operations in the i386_copy module, as BTX
maps the first 16M of memory. Check operations against the
detected size of actual memory.

Add BTX startup/interface code.

Add btxldr, a BTX loader for ELF clients.

This commit was generated by cvs2svn to compensate for changes in r39088,
which included commits to RCS files with non-trunk default branches.

BTX (aka the boot extender) is an i386 kernel that hosts 32-bit
bootstrap programs, and provides page-level protection, hardware
interrupt reflection, a virtual-8086 mode interface to BIOS, etc.