systemd: bump to 236, convert to meson

systemd: bump to 236, convert to meson

systemd is no longer an autotools package, as such, it has now been converted
over to meson.

Even though systemd234 has meson support, it is broken with gcc7, as such
the revision bump and conversion to meson must be in a single patch.

Changes include:
  - Change systemd from an autotools package to a generic package
  - Changing all the options from --enable/disable to -Doption=true/false
  - Remove --without-python (no longer an option)
  - Remove all of the ac_cv_path_ variables, and move them into CONF_OPTS
    with the prefix -Doption-path=/path.
  - Add sha256sum's for the license files.
  - Remove 0002-build-check-for-ln-relative.patch and add
    0002-install-dont-use-ln-relative.patch in its place, the old patch relied on autotools and is no longer relevant.
  - Add 0004-add-false-option-for-tests.patch. With the conversion to meson,
    systemd no longer has the option to disable unit tests from being built.
    This patch re-adds the functionality. This prevents 381 files from being
    built, and prevents gcrypt from becoming a dependency.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Tested-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: gitlab-ci https://gitlab.com/ymorin/buildroot-ci/pipelines/15857672/builds
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>