swupdate-lualoader: Avoid warning about missing S directory

In wrynose, bitbake started complaining about missing S directory at the
end of do_unpack.

WARNING: swupdate-lualoader-1.0-r0 do_unpack: swupdate-lualoader: the directory ${UNPACKDIR}/${BP} (/work/build/tmp-os/work/cortexa72-oe-linux/swupdate-lualoader/1.0/sources/swupdate-lualoader-1.0) pointed to by the S variable doesn't exist - please set S within the recipe to point to where the source has been unpacked to
Signed-off-by: Esben Haabendal <esben@geanix.com>

recipes-lua/swupdate-lualoader/swupdate-lualoader_1.0.bb

@@ -5,6 +5,7 @@ DEPENDS = "lua"
 LIC_FILES_CHKSUM = "file://${UNPACKDIR}/swupdate_handlers.lua;md5=354cf4af377edd962d2e8d78085d3ed7;beginline=1;endline=19"
 
 SRC_URI = "file://swupdate_handlers.lua"
+S = "${UNPACKDIR}"
 
 inherit pkgconfig