modules.sh can't depend on openwrt/feeds.conf.default, otherwise the
initial update will fail.

Using slashs instead of dots is a bit unexpected, and as long as we're not
going for full-blown JSONPath, neither is more correct than the other.

Based-on-patch-by: lemoer <git@irrelefant.net>

Each domain is validated separately, preferring domain values to site
values.

Based-on-patch-by: lemoer <git@irrelefant.net>

This does not do anything yet, as this_domain() is not implemented yet.

Based-on-patch-by: lemoer <git@irrelefant.net>

scripts/filesize.sh doesn't follow symlinks.

Fixes #1316

In addition to significant internal differences in check_site_lib.lua (in
particular unifying error handling to a single place for the upcoming
multi-domain support), this changes the way fields are addressed in site
check scripts: rather than providing a string like 'next_node.ip6', the
path is passed as an array {'next_node', 'ip6'}.

Other changes in site check scripts:
* need_array and need_table now pass the full path to the sub fields to the
subcheck instead of the key and value
* Any check referring to a field inside a table implies that all higher
levels must be tables if they exist: a check for {'next_node', 'ip6'} adds
an implicit (optional) check for {'next_node'}, which allows to remove many
explicit checks for such tables

The domain configs are not checked yet, and not used for anything.

Based-on-patch-by: lemoer <git@irrelefant.net>

These helpers are no-ops for now, until the actual multi-domain support is
added.

We would like to check the file size instead of downloading indefinitely.
The file size is added in another copy of the manifest lines, which is
ignored by older autoupdater implementations.

Fixes #1262

LEDE doesn't always notice that the metadata needs to be refreshed.

As the default package list is now handled using the 'packages' directive,
explicit wpad-mini removals in target definitions can be dropped.

This, together with a new try_config directive, will allow checking the
generic config and make it more flexible.

To reduce the number of packages that need to be listed in
GLUON_SITE_PACKAGES, this adds a new variable GLUON_FEATURES. Sets of
packages are enabled automatically based on the combination of listed
feature flags.

Site-specified package feeds can provide their own feature flag
definitions.

Fixes #1126

Don't stop after the first error, as later error messages may make the
cause of the error much clearer in some cases.

The current autoupdater will only respect the last line for a given model
name, so we can add SHA256 checksums as long as they occur before the
corresponding SHA512 line.

Setting DEVICES will also suppress cleaning the package directory,
improving incremental build speed.

The default abbreviation length depends on factors like user configuration,
git version and number of repository objects. Use unabbreviates IDs to make
patch generation more reproducible.

need_one_of(varname, array, required) checks weather the value of the specified variable is part of given array.
need_array_of(varname, array, required) is similar to need_one_of() but assume that varname points to an array.