package/systemd: make sure init choice and package have same dependencies

package/systemd: make sure init choice and package have same dependencies

Currently, the dependencies for the init system choice, and the
dependencies for the package, are slightly different, and not in the
same order, the latter making it difficult to assess consistency between
the two.

Fix all that, by cross-duplicating dependencies from the init choice and
the package, and order the dependencies according to the manual (arch
first, toolchain, then the others).

Note that some dependencies are redundant, but kept nonetheless for
correctness:

  - BR2_USE_MMU is implied by BR2_TOOLCHAIN_USES_GLIBC, but systemd does
    use fork();

  - !BR2_STATIC_LIBS is also implied by BR2_TOOLCHAIN_USES_GLIBC, but it
    is also inherited from kmod which we select;

  - BR2_TOOLCHAIN_HAS_THREADS is also implied by BR2_TOOLCHAIN_USES_GLIBC,
    but systemd does use pthread_*() functions.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>