lua-rs232: don't rely on detected luadir

lua-rs232: don't rely on detected luadir

If TOPDIR starts with /usr, then the configure script will use the
staging tree hierarchy instead of using plain /usr/lib.  For example, if
TOPDIR=/usr/src/openwrt, then the files will not be available under
$(PKG_INSTALL_DIR)/usr/lib/lua/5.1/, as expected, but under
$(PKG_INSTALL_DIR)/usr/src/openwrt/staging_dir/hostpkg/lib/lua/5.1/.

Set the correct path when calling 'make'.  As a bonus, the hardcoded
version number in the Makefile can be dropped.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>