Add a new release build variable, WITH_COMPRESSED_IMAGES.

Add a new release build variable, WITH_COMPRESSED_IMAGES.

When set to a non-empty value, the installation medium is
compressed with gzip(1) as part of the 'install' target in
the release/ directory.

With gzip(1) compression, downloadable image are reduced in
size quite significantly.  Build test against head@263927
shows the following:

 bootonly.iso:		64% smaller
 disc1.iso:		44% smaller
 memstick.img:		47% smaller
 mini-memstick.img:	65% smaller
 dvd1.iso:		untested

This option is off by default, I would eventually like to
turn it on by default, and remove the '-k' flag to gzip(1)
so only compressed images are published on FTP.

Requested by:	wkoszek
MFC After:	1 week
Sponsored by:	The FreeBSD Foundation