swupdate: do not drop CFLAGS

Dropping CFLAGS raises link errors when building with LUA. Just LDFLAGS
should be removed.

Signed-off-by: Stefano Babic <sbabic@denx.de>

recipes-support/swupdate/swupdate.inc

@@ -77,7 +77,7 @@ do_configure () {
 }
 
 do_compile() {
-  unset CFLAGS CPPFLAGS CXXFLAGS LDFLAGS
+  unset LDFLAGS
   oe_runmake swupdate_unstripped
   cp swupdate_unstripped swupdate