libelementary: only use libethumb if libedbus is available.

libelementary uses the ethumb_client library, but that one is only built
if libedbus is available. So fixup the --enable/disable-ethumb according
to the selection of BR2_PACKAGE_LIBEDBUS.

Fixes e.g.
http://autobuild.buildroot.net/results/14ef98da6b0632e24514fef696fae9a650c90c96



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>