Instead of never overwriting compiler-gcc.h if it exists, overwrite it
by default except if OVERWRITE_GCCH=0 is set in the environment.

Most kernels *do* need this file replaced and won't build without this
patch.

At least one newer kernel already includes the compiler support split
stuff, so don't overwrite it if it already exists.

The 5.0 kernel now includes -Werror= parameters in the makefile, this
changes downstreamkernel_prepare.sh to support them. Without this, the
script will replace, for example, -Werror=blah with =blah, which causes
the kernel compilation to fail.

Removes the need to cd $builddir before calling downstreamkernel_prepare
in a kernel APKBUILD prepare function.

We used to put a "compiler-gcc6.h" file inside every vendor kernel's
aport. This is redundant and only works with GCC6. With this patch,
a common "compiler-gcc.h" file will be used, that works independently
of the GCC version.

The file is from Decatf's android_kernel_samsung_p4 repository, where
it was taken from upstream Linux, but adjusted to work with older
vendor kernels (different #ifndef check at the top etc).

https://github.com/Decatf/android_kernel_samsung_p4/blob/bd0772df2132bf56cda78e7204facf434cd5aa40/include/linux/compiler-gcc.h

* Travis and Coveralls badges
* aports: instead of <https://github.com/postmarketOS>, use
  <https://postmarketos.org>
* References to full URLs to issues and pull requests replaced with
  a hash and the number
* grsec check: simplify error message, remove link to github issue
  (nobody is using that anymore anyway)

* Automatically generate a calibration matrix for libinput
  This takes the calibration matrix for wayland and divides the pixel
  offsets by the device width/height.
* Bump pkgrels of devices using devicepkg-dev
  This causes new packages to be generated, using the new devicepkg-dev
  version.

* Use devicepkg-dev by default in new device wizard
* Add link to reference wiki page