Bump LXC package from 1.1.5 to 2.1.0. Version 2.x includes many
improvements and optimizations.

https://linuxcontainers.org/lxc/news/

Some tools like lxc-top are rewritten in C and are no longer broken and
dependent on lua.

Signed-off-by: Marko Ratkaj <marko.ratkaj@sartura.hr>

seccomp was unconditionally disabled before.
Also a non-escaped newline in a place where it should be escaped
made the build process skip some CONFIGURE_ARGS.
Fix CONFIGURE_ARGS and add an option to allow building with seccomp.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>

Signed-off-by: Luka Perkov <luka@openwrt.org>