Fixes#1227.
This also updates the hash in linux-postmarketos-mainline, because the
hash changed upstream (they updated their git version server-side?).
* Using full diff skipping v4.15-rc1
cgit doesn't generate proper patches for binary files (one was added in
v4.15-rc1 and removed in v4.15-rc5 because it also introduced a new
build-time dependency which is now solved). For 4.16 the "old" source
lines can be re-added but until then a full diff skipping v4.15-rc1
is needed.
* Add patch from linux-next to fix the build on some machines
(sync-check.sh was not executable, see #950)
The conflict happens, when we're on rc-1, the old naming only contains the target version, so there will be two file named `postmarketos-linux-mainline-4.15-rc1.patch`: one that updates the kernel from the latest stable, and one empty.
