see https://gitlab.com/postmarketOS/pmaports/issues/105
partial revert of https://gitlab.com/postmarketOS/pmaports/commit/f2a27fc0dffa03533e349a60fcb5b2688a6e4c90

All kernels compile again, after the GCC 8 upgrade.

All 3.x kernels have been changed to use GCC 6 now, because we can't
say for sure that they boot with GCC 8 even if we made them compile
with that newer GCC. If someone wants to test a kernel for a newer
device which they can test, see the instructions on
<https://postmarketos.org/vendorkernel>.

The linux-sony-castor-windy 4.x kernel did not compile out of the box
with the latest GCC, so we set it to GCC 6 as well. This can probably
be fixed easily.

linux-teclast-x80-pro: removed Werror and updated to latest 4.14 kernel
from kernel.org, otherwise this would not compile anymore (tested with
both GCC 6 and 8, probably incompatibility with the latest binutils or
something).

This commit will take too long to compile in CI, so let's [skip ci].
I've compiled all kernels multiple times and fixed them up until all of
them were working again.

Related: #103

Network Manager does not support dhcpcd with IPv6. Fixes issue #45

(this text is copied too)
If a user has an editorconfig plugin for their editor,
it will enforce tabs instead of spaces for indentation
for APKBUILD files. It ignores other files in the repo.

I did it manually in the previous commit and replaced the wrong for
loop in gcc6-armhf. [skip ci]

I thought only having specific symlinks was enough, and it was for one
kernel. However, to make sure it works with all kernels, let's just add
symlinks for all the binutils.

Without this symlink, lots of messages like this one appear while
compiling kernels:

/bin/sh: gcc6-armv6-alpine-linux-muslgnueabihf-objdump: not found

GCC6 cross compiler packages, based on the latest pmbootstrap
aportgen code. Related: #103

With this commit, pmbootstrap!1699 and a few changes in a linux aport,
it finally compiled successfully with gcc6 \o/

Details described in #103.

* remove everything that causes conflicts when installing gcc6 and
  gcc6-armhf side by side
* fix up the existing "if cross compiling" code path to use the
  /usr/gcc6 prefix
* link binutils binaries to the locations they are expected to be
  by gcc (this took me hours to figure out....) as well as by the
  linux build system with CROSS_COMPILE
* link all gcc binaries to /usr/bin/gcc6-*

Based on the last version from Alpine, but installed to /usr/gcc6 and
with pretty much everything disabled except for the C compiler.

We can probably throw out a lot of patches, and make the APKBUILD much
smaller. But let's focus on making it possible to compile old kernels
again first. There's still a lot to do for that, see #103.

Skipping CI, because it would probably not finish building all of them.
Major breakage incoming. Details discussed here:
<https://gitlab.com/postmarketOS/pmaports/issues/103>

I've missed the patch files in the previous commit, my bad.

This reverts commit 71ffb785.

This test case only makes sense in the pmbootstrap repository, and
we have it there already. In pmaports, we don't need to check if the
"aportgen" code is working - and to check the versions of the aports
we already have the test_upstream_compatibility case.

Got the info from:
<https://developer.puri.sm/Environments/PlasmaMobile.html>