All defined features need to be known at the same time, otherwise handling
a feed-provided feature definition file would add gluon-web-advanced etc.
to the package list when the corresponding feature flags appear in
GLUON_FEATURES.

Fixes: ee5ec5af ("build: rewrite features.sh in Lua")

The rewrite of the feature handling introduced multiple major bugs. One
of them was caused by the way Lua's logical operators work:

An expression of the form

    _'autoupdater' and _'web-advanced'

would return 'web-advanced' rather than the boolean true when _ returned
both strings unchanged (because the features are enabled).

As entries with more than a single feature name in their expressions did
not set no_default, Gluon would then attempt to add gluon-web-advanced to
the package selection, as web-advanced is a "pure" feature.

To fix this, and get rid of the annoying nodefault, separate handling of
"pure" feature and handling of logical expressions into two separate
functions, called feature() and when(). To simplify the feature
definitions, the package list is now passed directly to these functions
rather than in a table with a single field 'packages'.

Fixes: ee5ec5af ("build: rewrite features.sh in Lua")

Print a proper error message, rather than just

    openwrt/staging_dir/hostpkg/bin/lua: scripts/feature_lib.lua:48: bad
    argument #1 to 'setfenv' (integer expected, got nil)

The `features` file is converted to a Lua-based DSL.

A helper function `_` is used in the DSL; this will return the original
string for enabled features, and nil for disabled features. This allows
to use boolean operations on features without making the code too
verbose.

Besides having more readable and robust code, this also fixes the bug
that all files `packages/*/features` were evaluated instead of only
using the feature definitions of currently active feeds.