systemd: add PACKAGECONFIG to select default compression format

If multiple compression libraries are enabled, the format to use for
compression is chosen implicitly, so if you want to compress with e.g.
lz4 you cannot enable zstd, so you cannot read zstd-compressed journal
files. This PACKAGECONFIGs allow to enable all compression formats for
reading, but to choose a specific one for writing.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>