makechrootpkg: Allow setting rw layer dir name

Make the name of the rw layer (default <chrootdir>/rw) configurable
with a command line switch.

Useful for maintaining multipl chroots on top of a clean base.

i.e.
<chrootdir>/root  #clean chroot
<chrootdir>/rw    #default RW layer
<chrootdir>/gnome #a RW layer with all of gnome installed
<chrootdir>/kde   #a RW layer with all of KDE installed

Signed-off-by: Aaron Griffin <aaronmgriffin@gmail.com>