rngd: new module running early during boot to help generating entropy

rngd: new module running early during boot to help generating entropy
when system's default entropy sources are poor (e.g. use of SSD disks or
UEFI RNG not available)

On systems with low entropy at boot, the boot can take up to several
hours, specially when NBDE is used (e.g. clevis) which makes use of
the random number generator.

Enabling rngd service at boot early, because dracut-initqueue runs,
enables to initialize the random number generator in a couple of seconds
instead of minutes or hours.

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>