Update Buildroot (and everything)

There are several important updates:

- Buildroot now contains patches for per-package shared dirs, which is a
prerequisute for top-level parallel build. We're making heavy use of
that one.

- Buildroot has some semi-broken support for sysrepo. I've sent our
improvements upstream earlier today. Some changes are a bit more
granular (the C++ bindings for example).

- This also updated a bunch of packages, of course.

- Our systemd patch required updating, let's see if this one works well
under all circumstances.

Also update some docs as we're rebuilding anyway...

Change-Id: I816fc6105eb999b7b582527ff3540d7a6d634679
diff --git a/package/cla-sysrepo/Config.in b/package/cla-sysrepo/Config.in
index 1b8d59b..873a9aa 100644
--- a/package/cla-sysrepo/Config.in
+++ b/package/cla-sysrepo/Config.in
@@ -1,6 +1,8 @@
 config BR2_PACKAGE_CLA_SYSREPO
 	bool "cla-sysrepo"
 	select BR2_PACKAGE_SYSREPO
+	select BR2_PACKAGE_SYSREPO_CPP
+	select BR2_PACKAGE_LIBYANG_CPP
 	select BR2_PACKAGE_DOCOPT_CPP
 	select BR2_PACKAGE_SPDLOG
 	select BR2_PACKAGE_SYSTEMD