image-customization.lua has replaced GLUON_FEATURES and
GLUON_SITE_PACKAGES, so the file is always required to build a useful
Gluon image. In addition, not only 'file not found' errors were ignored,
but also all other Lua load errors, resulting in a build with empty
image customization when the file was not valid.

Closes #3218

Load the site image-customization.lua file only on init. Previously, the
file was loaded on every device invocation.

Signed-off-by: David Bauer <mail@david-bauer.net>

Create an init routine which loads the environment only once instead
every evaluation.

Signed-off-by: David Bauer <mail@david-bauer.net>

Include the file-extension with the image-customization.lua file. This
will ease work with editors providing syntax highlighting, as they now
properly detect the file-type.

Signed-off-by: David Bauer <mail@david-bauer.net>

If we align the table keys of the selection table to match the return
object of get_selections, we can omit creating a new return object.

Signed-off-by: David Bauer <mail@david-bauer.net>

This allows the negation of already activated features oder packages by
Gluon using the image-customization.

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: David Bauer <mail@david-bauer.net>

Implement a flexible system for handling site-defined features as well
as packages.

This system is inspired by the existing feature-system and allows for a
more flexible approach for selecting specific packages for devices.

Features are now defined in a `features` file in the site-root. The same
goes for packages.

These files are sequentially evaluated and the device-package list is
evaluated for each device independently.

Signed-off-by: David Bauer <mail@david-bauer.net>