loader: add target for dirdeps build

Update dependencies for the loader variations used for each
architecture.

Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D39741

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

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

Sponsored by: Netflix