We currently don't have any deprecated devices, so it doesn't make much
sense to force every site to specify this variable. Make it default to 0
instead.

OpenWRT 19.07 enables urngd by default, so haveged is redundant.

Fixes #1775

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

Closes: #1436