package/systemd: add host variant

package/systemd: add host variant

Add the infrastructure to build the host version of systemd
* disable all optional features, they can be re-added when needed
* systemd has creative way of dealing with cross compile
  we build a "normal" host systemd, but install it in $HOST_DIR
  we use systemctl --root to correctly act on TARGET_DIR
* we need to adjust RPATH using patchelf because meson can't do it
  correctly by itsel

The first question is: why do we use --prefix=/usr ?

systemd will store its --prefix in all the executables it generates. As
such, systemctl will have a hardcoded 'prefix', where it will manipulate
and create files/symlinks in. When called natively, this is nice and
shinny.

However, for cross-setup, that does not work obviously.

So, systemd has its tools know about the 'root' directory where this
prefix should be related to. We can call systemctl --root=$(TARGET_DIR)
and systemctl wil do the links and such in there.

However, it does so by appending its known prefix to it.

So, if we were to configure host-systemd as we usually do, with
--prefix=$(HOST_DIR), then when we would call host systemctl --root=$(TARGET_DIR)
it would look for files in $(TARGET_DIR)/$(HOST_DIR), which is wrong.

Calling the host systemctl without --root is also wrong, as it would look for
files in $(HOST_DIR)

So, there is no satisfying official support for this case.

The trick then, is to configure systemd with the prefix it would expect
at runtime (on the target!), that is with /usr, but install out-of-tree.

That was it for the first part of the question: why do we use --prefix.

Now, the second question is: why do we need to muck up with the rpath
after installation?

Well, this boils down to meson (and not systemd itself). When it
installs executables, meson will handily insert whatever rpath the
package meson.build would tell it to use. systemd installs libs in
$(prefix)/lib/systemd and has a NEEDED to those libs, so it uses an
RPATH to find those libs, and meson does inject that RPATH into the
installed executables.

However, we Buildroot also want to insert our own RPATH, because systemd
uses util-linux' libs and libcap, installed in $(HOST_DIR), so it needs
our RPATH.

However, meson can not extend the RPATH from the LDFLAGS in the
environment; meson can only set the RPATH from what it knows about from
the package's meson.build.

That, in addition to the --prefix=/usr issue above, means that the
executables installed by host-systemd have an RPATH set to
/usr/lib/systemd. when we would want it to be set to
$(HOST_DIR)/lib:$(HOST_DIR)/lib/systemd

That's what is done in the post-install hook: set the RPATH to the
appropriate values.

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
[yann.morin.1998@free.fr:
  - reformatting in commit log
  - declare host variant after target variant
  - simplify comments
  - slight reordering of variable (HOST_SYSTEMD_NINJA_ENV moved)
  - reformatting for mutli-line variable (HOST_SYSTEMD_HOST_TOOLS)
  - don't split HOST_SYSTEMD_CONF_OPTS in two sets
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>