buildroot

libarchive: avoid some tools to build statically on shared builds

Some libarchive tools, like bsdtar, builds statically by default
although libarchive itself is being built dynamically. We can force
those tools to build the same way as libarchive by using the right
configure options.

Fixes:
  http://autobuild.buildroot.net/results/b8f/b8f7a29787ea1cc5c98e4cbd5f47f257f9b306f2/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>