add uci option to set "update_packages". this options will lead the
luci-app-attendedsysupgrade to tell the update server to check for
package updates as well (not only release upgrades)

Signed-off-by: Paul Spooren <paul@spooren.de>

use named uci options

Signed-off-by: Paul Spooren <paul@spooren.de>

add ubus call to perform a sysupgrade and acl file for the attended
sysupgrade use case as well uci defaults.
Package is a part of the GSoC 17 project implementing easy
sysupgrade functionality.

Signed-off-by: Paul Spooren <paul@spooren.de>