Rename the 'system' target to 'disc1', which is specific
to the disc1.iso installer image.

Replace 'system' with 'release' in CLEANFILES, and add
'disc1' to CLEANDIRS.

Ensure the 'dvd' target depends on 'packagesystem'.

Fix 'mini-memstick.img' prerequisite, which should be the
'bootonly' target, not 'disc1' (previously 'system').

Use .TARGET as the target installation directory for the
disc1.iso and bootonly.iso images, which now expand to
'disc1' and 'bootonly' respectively, mimicking the behavior
of the 'dvd' target.

Remove '@true' from the 'release' target, and instead use
'touch ${.TARGET}' to prevent multiple iterations of
'make release' from clobbering previously-built installer
medium.

Tested with: head@r277834
MFC after: 3 weeks
X-MFC-with: r277458, r277536, r277606, r277609
Sponsored by: The FreeBSD Foundation