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.

MFC r313982, r314068:
sys: Replace zero with NULL for pointers.

Found with: devel/coccinelle

MFC r307518:
Fix device delete child function.

When detaching device trees parent devices must be detached prior to
detaching its children. This is because parent devices can have
pointers to the child devices in their softcs which are not
invalidated by device_delete_child(). This can cause use after free
issues and panic().

Device drivers implementing trees, must ensure its detach function
detaches or deletes all its children before returning.

While at it remove now redundant device_detach() calls before
device_delete_child() and device_delete_children(), mostly in
the USB controller drivers.

Tested by: Jan Henrik Sylvester <me@janh.de>
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D8070

MFC r305804:
Make the callout structure in the boot loader's kernel shim more
similar to the kernel one.

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

Provide busdma stubs for loader/kshim

Simple bus space stubs require the VA-PA mapping
to be identical.

Approved by: hselasky, cognet (mentor)
Differential revision: https://reviews.freebsd.org/D4314

Add support for exynos5_ehci in loader

Create new driver which initializes Arndale PHY and calls ehci_init

Reviewed by: hselasky
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4192

Run callouts during infinite waiting inside cv_wait

During cv_wait we may be waiting for an event triggered by callout.
Run callbacks here to avoid code blocking.

Reviewed by: hselasky
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4144

Merge from CheriBSD:

Make the sysinit tool a build tool rather than building in with
/usr/bin/cc and running it from OBJDIR. (It will be moved to usr.bin
once a manpage is written and a few style cleanups are done.)

Split the makefile bits for Hans' kernel shim layer into their own
includable kshim.mk.

Move USB support into a .mk file so loaders can include it.

USB boot library improvements:
- Make the USB boot library more configurable.
- Resolve compile issues when cross building.
- Allow use of separate malloc.
- Allow use of separate endian macros.

Sponsored by: DARPA, AFRL

Fix build after recent DWC OTG changes.

Add defines to more easily allow a single threaded version of the FreeBSD
USB stack. This is useful for non-kernel purposes, like the loader.

Initial version of libusbboot, a fully stand-alone, single threaded and
functional compilation of the FreeBSD USB stack for use with boot loaders
and such.

Discussed with: Hiroki Sato, hrs @ EuroBSDCon