Makefile: fix static build

Created by: ffontaine

Don't build and install lua_swupdate.so if HAVE_LUA isn't set to y otherwise build will fail if the toolchain only suports building static libraries

Fixes:

• http://autobuild.buildroot.org/results/c11c4d26983e0347d96f3dda62e6d72b031967bb

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com