Allow overriding the default selection state for Devices, similar to
setting a default for packages.

E.g. by setting DEFAULT to n, they won't be selected by default anymore
when enabling all device in the multi device profile.

This allows preventing images being built by the default config for
known broken devices, devices without enough RAM/flash, or devices not
working with a certain kernel versions.

This does not prevent the devices from being manually selected or images
being built by the ImageBuilder. These devices often still have worth
with a reduced package-set, or as a device for regression testing, when
no better device is available.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>