Create a _pkgbootstrap target, and remove the env(1) prefix

Create a _pkgbootstrap target, and remove the env(1) prefix
to pkg(8) commands.

Move the resulting packages outside STAGEDIR to minimize
pollution.

When invoking 'pkg create', include the ABI in the REPODIR
path so the correct hierarchy is created for cross-builds.

Move the STAGEDIR and REPODIR declarations above the targets
that use them to keep things cleaner, and move the packages
target.

Include '-o ABIFILE=DESTDIR/bin/sh' in pkg(8) invocations in
the create-kernel-packages target.

Sponsored by:	The FreeBSD Foundation