MFC r261302:

The -B flag is intended to take an argument.

Fix a couple typos in comments.

Sponsored by: DARPA, AFRL

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

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

mdoc: silence a few mandoc lint warnings.

Cleanups and markup improvements suggested by trociny and joel.

Minor mdoc improvements.

Add a sample script to create filesystem images from an installed tree
created by installworld+distribution with the -DNO_ROOT option.

Add support for an external cross compiler. The cross compiler is
specified by passing the XCC, XCXX, and XCPP variables (corresponding to
CC, CXX, and CPP) to buildworld/buildkernel. The compiler must be clang
or be configured to target the appropriate architecture.

To speed build times, if XCC is an absolute path or
WITHOUT_CROSS_COMPILER is defined then no cross compiler will be built
during the cross-tools stage.

Limited documentation of this feature can currently be found at:

https://wiki.freebsd.org/ExternalToolchain

This functionality should be considered experimental and is subject to
change without notice.

Sponsored by: DARPA, AFRL
Discussed with: imp, sjg