- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

add detection of serial console presence to btx and boot2-like blocks

Note that this commit slightly increases size of boot blocks.

Reviewed by: jhb
Tested by: Olivier Cochard-Labbe <olivier@cochard.me>
MFC after: 26 days

Build boot2 with -mregparm=3, ie. pass upto 3 arguments via registers.
This modifies CFLAGS and tweaks sio.S to use the new calling convention.
The sio_init() and sio_putc() prototypes are modified so that other
users of this code know the correct calling convention.

This makes the code smaller when compiled with clang.

Reviewed by: jhb
Tested by: me and Freddie Cash <fjwcash gmail com>

Add the ability to specify the boot2 serial console speed in
/boot.config or on the "boot:" prompt line via a "-S<speed>" flag,
e.g. "-h -S19200". This adds about 50 bytes to the size of boot2
and required a few other small changes to limit the size impact.
This changes only affects boot2; there are further loader changes
to follow.

$Id$ -> $FreeBSD$

Add serial, dual, and probe-keyboard support.