init: T2044: only start rpki if cache is configured

This extends commit 9199c87cf ("init: T2044: always start/stop rpki during
system boot") to check the bootup configuration if an RPKI cache is defined.
Only start RPKI if this is the case.

(cherry picked from commit 9b8e11e078c42e3ae86ebfa45fec57336f25a0af)