luci-app-attendedsysupgrade: add package

Add a simple luci view in system tab to perform a sysupgrade
with no hassle.

Asks an external update server for upgrades, downloads and
flashes the upgrade.

This packge is the frontend for my current GSoC project:
https://summerofcode.withgoogle.com/projects/#5684958571003904

More code is found here:
https://github.com/aparcar/gsoc17-attended-sysupgrade

Signed-off-by: Paul Spooren <paul@spooren.de>
[jo@mein.io: squash commits, remove PKGARCH from Makefile]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>