batctl: new package

[Thomas:
 - remove trailing whitespace
 - remove "# libnl" comment in Config.in
 - remove BATCTL_SOURCE, which is not needed, since the default value
   is used
 - pass $(TARGET_CONFIGURE_OPTS) in the make environment, not as a
   make option, otherwise the CFLAGS += lines of batctl Makefile do
   not have any effect, and lead to not have the proper include path
   to the libnl header files.]

Signed-off-by: Jens Zettelmeyer <zettelmeyerj@goooglemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>