Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Thanks to David's patches, we can now simplify these makefiles
further, and just use PROG directly (without FILES).

Suggested by: bde

Properly merge boot0sio.s and boot0.s into boot0.S.

First round of cleanups to sys/boot/ makefiles:

- do not use PROG for what's not a real C program,
- use sys.mk transformation rules where possible,
- only create the "machine" symlink on AMD64,
- removed MAINTAINER lines in individual makefiles,
- added the LIBSTAND defitinion to <bsd.libnames.mk>,
- somewhat better contents in .depend files.

Tested on: i386, amd64
Prodded by: bde

Hook boot0sio to the build. Use 'boot0cfg -b /boot/boot0sio' to use.