Cherry-pick the following commits from master to bump opkg in LEDE 17.01:

b65dc04712 opkg: switch to own fork to improve memory usage
55ffc38004 opkg: re-enable usign support
19720a6f03 opkg: fix handling conffiles in status lists
9e4555f58d opkg: fix stray printf() (#551)
ebf846b005 opkg: mark as essential (FS#571)
aedd5d5cb0 opkg: fix several package installation bugs
48ae44d033 opkg: gracefully handle missing $PATH, fix build warnings
1449b52f02 opkg: backport upstream fixes, code cleanups

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

Clarify opkg's messages related to downloads:

* more visible error message for package list download failure
* separate error message for signature file download error
* if wget returns 4, signal the network error more clearly
* remove '.' from end of filenames and URLs

* try signature check only if the package list was downloaded ok.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

opkg doesn't have BUILD_VARIANTs anymore, so the previously defined
PKG_BUILD_DIR would lead to a weird 'opkg-' path component.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

It has never been used by default (due to being too bloated), and it is
properly replaced by usign (which has been the default for a long time
now).

Remove this feature to simplify the build system

Signed-off-by: Felix Fietkau <nbd@nbd.name>

Opkg's builtin decompression code is unsuitable to process nested archives as
it uses a single shared state and relies on undefined seek behaviour for pipes.

Rework the extraction logic to use the external gzip command as I/O filter for
decompressing data and remove the builtin inflate code entirely.

This shrinks the final opkg binary by about 4KB and results in less runtime
memory consumption due to efficient use of vfork() and less copy-on-write
operations in the forked child.

Rework by Felix: create a thread that relays data to the gzip process
instead of using a fragile poll loop

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

This reverts commit 0090adcd5c94adad2168cd9b338f45827533c81d.
It breaks reading package list in /tmp/opkg-lists, making it impossible
to install packages from feeds in snapshots.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

Opkg's builtin decompression code is unsuitable to process nested archives as
it uses a single shared state and relies on undefined seek behaviour for pipes.

Rework the extraction logic to use the external gzip command as I/O filter for
decompressing data and remove the builtin inflate code entirely.

This shrinks the final opkg binary by about 4KB and results in less runtime
memory consumption due to efficient use of vfork() and less copy-on-write
operations in the forked child.

Rework by Felix: create a thread that relays data to the gzip process
instead of using a fragile poll loop

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

Also use default defintions for PKG_SOURCE_SUBDIR, PKG_SOURCE

Signed-off-by: Felix Fietkau <nbd@nbd.name>

They are based on obsolete download tarballs

Revert "usign: fix mirror hash" (4f874423fd711e58aa43110918c923016615f22b)
Revert "ubox: fix mirror hash" (9ee74b575305e8111fd3b1009ae183cf15e6d926)
Revert "opkg: fix mirror hash" (bf6f4bfad12ded13ed9afcb886751cec24426257)

Signed-off-by: Felix Fietkau <nbd@nbd.name>