release: Don't reuse disc1/bootonly directories

The disc1 and bootonly directories have files distributed into them
for use in "full" and "mini" images; the former are disc1.iso and
memstick.img, and the latter is bootonly.iso and mini-memstick.img.

Unfortunately the scripts which package a directory tree into an ISO
or memory stick image also modify the directory, for example to
create an appropriate /etc/fstab file; so creating two images at the
same time breaks.

Resolve this by copying disc1 to disc1-disc1 and disc1-memstick,
and copying bootonly to bootonly-bootonly and bootonly-memstick,
before using those directories for constructing the ISO+memstick
images.

MFC after: 1 week

(cherry picked from commit a1bff97300abe4b46528357e39c83da764df1fd8)