boost: fixup host variant dependencies

Define an empty HOST_BOOST_DEPENDENCIES to avoid inheriting
BOOST_DEPENDENCIES.
Also ditch redundant empty definitions for BOOST_DEPENDENCIES and
BOOST_FLAGS.

Fixes:
http://autobuild.buildroot.net/results/9c6/9c64dd473a8096bb6343d746544997e333053e8b/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>