MFC r325834,r325997,326502: Move sys/boot to stand/

This is effectively a direct commit to stable/11, due to differences between
stable/11 and head. Changes to DTS in sys/boot/fdt/dts were often
accompanied by kernel changes. Many of these were also risc-v updates that
likely had many more dependencies to MFC.

Because of this, sys/boot/fdt/dts remains as-is while everything else in
sys/boot relocates to stand/.

r325834: Move sys/boot to stand. Fix all references to new location

r325997: Remove empty directories.

r326502: Document the sys/boot -> stand move in hier.7 and the top-level README.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Add kexec-based Linux shim loader for use with the firmware petitboot
loader on PS3 and POWER8 systems. This is reasonably portable to other
architectures, especially FDT-based ones, if similar features are useful
elsewhere.

Netboot support is missing for now and will be added in a future commit,
at which time loader.ps3 will be garbage collected.

Done at: Hackathon

Port to Linux.

Make countdown, and interrupting the countdown, work.

Add block device support. This can't autoload the kernel for some reason,
but can manually. Not sure why.

Add some initial bits, enough to print out the loader greeting. This uses
FreeBSD syscalls for now, but with changes to the numbers, it also works on
Linux.

Disk and network IO are the next steps.