contrib: add push_pkg.sh to enhance package development workflow (v2) (#2023)

v2: In contrast to the last patches, this is now built on top of ssh
    only, without using e.g. 9pfs. Furthermore it works also with
    arbitary remote hosts on any target/architecture. Also the
    scripts were renamed and moved to /scripts.

The aim of this commit is to allow fast rebuild cycles during the
development of gluon packages.

Currently the following workflow can be used:

	# start a local qemu instance
	scripts/run_qemu.sh output/images/factory/[...].img

	# do your changes in the file you want to patch
	vi package/gluon-ebtables/files/etc/init.d/gluon-ebtables

	# rebuild and update the package
	scripts/push_pkg.sh package/gluon-ebtables/

	# test your changes
	...

	# do more changes
	...

	# rebuild and update the package
	scripts/push_pkg.sh package/gluon-ebtables/

	# test your changes
	...

	(and so on...)

Implementation details:
- Currently this is based on ssh/scp.
- Opkg is used to install/update the packages in the remote machine.

Benefits:
- This works with compiled and non-compiled packages.
- This works with native OpenWrt and Gluon packages.
- This even performs the check_site.lua checks as they are integrated
  as post_install scripts into the openwrt package.
- It works for all architectures/targets.