purge_single command can be usefull to save space during build. It removes
all files in working directory except "iso/" and *.iso.

Workflow can be:

(32)  ./build.sh all_iso_single -> i686 *.iso are ready.
(32)  ./build.sh purge_single -> safe cleanup.
(64)  ./build.sh all_iso_single -> x86_64 *.iso are ready.
(64)  ./build.sh purge_single -> safe cleanup.
(any) ./build.sh all_iso_dual -> dual *.iso are ready.

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>