meson: use jinja2 for rpm templates

The naming of variables is very inconsistent. I tried to use more
modern style naming (UNDERSCORED_TITLE_CASE), but I didn't change existing
names too much. Only SYSTEM_DATA_UNIT_PATH is renamed to SYSTEM_DATA_UNIT_DIR
to match SYSTEM_CONFIG_UNIT_DIR.

src/debug-generator/debug-generator.c

@@ -134,7 +134,7 @@ static int generate_wants_symlinks(void) {
                 if (!p)
                         return log_oom();
 
-                f = path_join(SYSTEM_DATA_UNIT_PATH, *u);
+                f = path_join(SYSTEM_DATA_UNIT_DIR, *u);
                 if (!f)
                         return log_oom();