openswan: add missing TARGET_MAKE_ENV

openswan needs host-bison. The dependency was already there, but the
build was not executed with $(TARGET_MAKE_ENV) in the environment, and
therefore $(HOST_DIR)/usr/bin was not in the PATH.

While we're at it, add licensing informations to openswan.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>