agl-profile-core/packagegroup-agl-core-boot: add rng-tools

agl-profile-core/packagegroup-agl-core-boot: add rng-tools

On some boards (typically Minnowboard Max), the kernel's random number entropy
pool may fill too slowly.  As a consequence, if many processes ask for random
data at the same time, the reads on /dev/random are blocked until the entropy
pool has been filled.

This patch adds rng-tools in the core images: the rngd daemon will start at boot
and fill the entropy pool.

Bug-AGL: SPEC-1655

Change-Id: Icbcf6a9ea685774dd13e33597689a3fd05aaadf4
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>