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>

Now we use the new Image-customization framework in Gluon, we need to
also update the docs so examples and descriptions are acurate again.

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

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.

v2: In contrast to the last patches, this is now built on top of ssh
    only, without using e.g. 9pfs. Furthermore it works also with
    arbitary remote hosts on any target/architecture. Also the
    scripts were renamed and moved to /scripts.

The aim of this commit is to allow fast rebuild cycles during the
development of gluon packages.

Currently the following workflow can be used:

	# start a local qemu instance
	scripts/run_qemu.sh output/images/factory/[...].img

	# do your changes in the file you want to patch
	vi package/gluon-ebtables/files/etc/init.d/gluon-ebtables

	# rebuild and update the package
	scripts/push_pkg.sh package/gluon-ebtables/

	# test your changes
	...

	# do more changes
	...

	# rebuild and update the package
	scripts/push_pkg.sh package/gluon-ebtables/

	# test your changes
	...

	(and so on...)

Implementation details:
- Currently this is based on ssh/scp.
- Opkg is used to install/update the packages in the remote machine.

Benefits:
- This works with compiled and non-compiled packages.
- This works with native OpenWrt and Gluon packages.
- This even performs the check_site.lua checks as they are integrated
  as post_install scripts into the openwrt package.
- It works for all architectures/targets.

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")

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.

$(TOPDIR) does not contain a trailing slash.
Thus the 'gluon.mk'-include must include
the trailing slash. (Just like the link to
$(INCLUDE_DIR)/package.mk does.)

Signed-off-by: Chris Fiege <chris@tinyhost.de>

Explain the magic of gluon.mk. The feature flag documention is moved into
this new page.