MFC r282785, r282787, r282789, r282790, r282791, r282792, r282793,
r282794, r282795, r282796, r282797, r282798, r282799, r283265,
r283266, r283268, r283271, r283272, r283298, r283302:

Add a new file, Makefile.mirrors, which is intended to replace
a 474-line kludge of a shell script to pre-create the directory
hierarchy on ftp-master.

This is not in any way connected to the build, and there is no
intention to do so. This only intent here is to try to make
things a little bit easier.

Always use the 'make install' directory as the source for images.
For RE purposes, we use the default (/R within the chroot), so
this helps avoid copying files multiple times and xz(1)-compressing
additional times when not needed.

Fix staging virtual machine images for ftp propagation.

This Makefile is not for general consumption.

Sponsored by: The FreeBSD Foundation