into the FreeBSD boot loader, typically for non-USB aware BIOSes, EFI systems
or embedded platforms. This is also useful for out of the system compilation
of the FreeBSD USB stack for various purposes. The USB kernel files can
now optionally include a global header file which should include all needed
definitions required to compile the FreeBSD USB stack. When the global USB
header file is included, no other USB header files will be included by
default.

Add new file containing the USB stack configuration for the
FreeBSD loader build.

Replace some __FBSDID()'s by /* $FreeBSD$ */ comments. Now all
USB files follow the same style.

Use cases:
 - console in loader via USB
 - loading kernel via USB

Discussed with:		Hiroki Sato, hrs @ EuroBSDCon