Use makefs(8) in release VM-image generation instead of md(4) and newfs.

Using makefs instead reduces the privileges needed to build VM images,
simplifies the script (no need to copy files to a fresh image at the end),
and improves portability by allowing generation of cross-endian images.
As a result of the last, this patch also adds support for generation of
powerpc64 and powerpc64le VM images.

No other changes to the output. Tested and working for both amd64 and
powerpc64 targets.

Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D28912

(cherry picked from commit 1ca8842f3ad9725863c9affc044d1974a51818a9)