Fixes: #1463

The new extra_image command allows to copy additional images that don't
fall into the sysupgrade or factory categories to the output directory.
These images will be placed in the "other" subdirectory.

The extra_image command takes 3 arguments: in_suffix (suffix in OpenWrt
bin directory), out_suffix (suffix in Gluon output directory) and ext (file
extension). Example:

  extra_image '-tftp' '-tftp' '.bin'

The ext argument can be omitted to control the whole filename suffix
including the extension from in_suffix and out_suffix. Multiple extra_image
commands can be used for a single device; aliases work as usual.

Based-on-patch-by: David Bauer <mail@david-bauer.net>

a7b2764879db include/prereq-build.mk: explicitly check for -f flag when using busybox time

bc7abe8729f3 include/kernel-build.mk: fix kernel rebuild on backport patch changes
c99c70e05303 kernel: backport page fragment API changes from 4.10+ to 4.9

Do not break the build on kernel 4.9 (ar71xx).

3e81365141e4 kernel: gen_stats: Fix netlink stats dumping in the presence of padding
dd1f97b37d41 ramips: clean up and fix MT7621 NAND driver issues
68f40d7ecc4c mt76: update to the latest version
16a9ddfe64e2 ramips: ethernet: use own page_frag_cache
0e6cfb691914 ramips: ethernet: use skb_free_frag to free fragments

514a4b3e1b4e include/feeds.mk: rework generation of opkg distfeeds.conf
3e89f58a5ead base-files: fix feed list in PKG_CONFIG_DEPENDS
7fa7c854fe33 include/feeds.mk: always add available feeds to PACKAGE_SUBDIRS
a5b9553cf592 scripts/feeds: add src-dummy method
2f306873efbe ramips: TP-Link TL-WR902AC v3: add missing wps button
36a4681b2bef ramips: TP-Link TL-WR902AC v3: don't build factory image
ca903c73c705 kernel: bump 4.14 to 4.14.54 for 18.06
3a5498c5e55d kernel: bump 4.9 to 4.9.111 for 18.06
27014da237f1 mac80211: Expose support for ath9k Dynack
5a40fad22a39 kernel: bcm47xxpart: fix getting user-space data partition name

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

be2c35785994 respondd-module-airtime: Fix reported noise result (#190)

add a missing command to set the special Gluon MAC address of the private WLAN interface to the documentation.

With all the packages being built 10GB are not enough anymore.

Closes: #1436

This commit makes use of the Power-LED as Diag-LED, allowing the LED to
work as a status indicator for config-mode.

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

Let sites leave these texts empty by default, just add a comment where to
find the default labels.

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.

The dependency line was lost during the rebase of the current master
version.

Fixes: 9d719a2e ("ar71xx: add support for OpenMesh A40/A60 (#1424)")

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