openvswitch: only libopenvswitch depends on libunbound, libunwind

openvswitch: only libopenvswitch depends on libunbound, libunwind

This is to make explicit the conditional select of libunwind is only
needed for libopenvswitch.  If we spill it over other packages, the
generated kconfig will have recursive dependency issue.  2 new patches
were made for this goal

The other thing is that "+libunwind" will cause it to be built if any of the
packages defined in this Makefile is enabled (y or m).  This is at the moment
by-design of the build system.

Libunwind does not support architectures like arc.  Use conditional select To
avoid (libunwind) build failures like the following,

  checking for ELF helper width... configure: error: Unknown ELF target: arc
  make[3]: *** [Makefile:65: /data/openwrt/build_dir/target-arc_arc700_uClibc/
  libunwind-1.3.1/.configured_68b329da9893e34099c7d8ad5cb9c940] Error 1

Things like "+PACKAGE_openvswitch-libopenvswitch:libunwind" will also result in
recursive deps error for chains of 3 nodes.  Kconfig construct like the
following will be made

	config A
		tristate
		select B
		depends on !(C) || (x)

	config B
		tristate
		select C

	config C
		tristate

	config x
		bool

Other changes include

 - Shared use of variable ovs__common_depends was removed
 - Ovn doc build was patched out

Link: https://github.com/openwrt/packages/pull/12959#issuecomment-665021413
Reported-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>