getrandom blocks until the random pool is being initialized.
Unfortunately, this code is being called early during init to create the
overlay filesystem, on some devices leaving little chance for a
successful random pool init.
True randomness is not that important here, so fix this issue by
sticking to using /dev/urandom, like in older versions of this code.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
