Fixes FS#17182: copytoram=y does not work with aufs

The solution is simple, just use a directory outside aufs tree.

* If copytoram=y, then another tmpfs is mounted but on /tmpfs.sqfs
* Add a new option: tmpfs_sqfs_size= (default to 75%)
* Lock cdrom drive is not locked if copytoram=y

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