[build] allow disabling systemd service unit install

Add `INSTALL_SYSTEMD_UNIT` which can explicitly be set to `false`
to disable installing the service unit even if systemd is available.

If not defined, it defaults to true, i.e. the systemed service unit
is created. As a result, this change is backwards compatible: it's
required to _opt-out_ of systemd unit installation.

CMakeLists.txt

@@ -104,7 +104,7 @@ include(GNUInstallDirs)
 
 pkg_check_modules(SYSTEMD systemd)
 
-if(SYSTEMD_FOUND)
+if(SYSTEMD_FOUND AND (NOT DEFINED INSTALL_SYSTEMD_UNIT OR INSTALL_SYSTEMD_UNIT))
     pkg_get_variable(OTBR_SYSTEMD_UNIT_DIR systemd systemdsystemunitdir)
 endif()