samples: smp_svr: fix overlay-udp.conf build

- fixes overlay-udp.conf build for platforms without TRNG.
- adds TEST_RANDOM_GENERATOR, as for most network samples.

Signed-off-by: Andrej Butok <[email protected]>
(cherry picked from commit 5c376ad)

Author: butok

samples/subsys/mgmt/mcumgr/smp_svr/overlay-udp.conf

@@ -11,5 +11,6 @@ CONFIG_NET_IPV6=y
 CONFIG_NET_SOCKETS=y
 CONFIG_NET_CONNECTION_MANAGER=y
 CONFIG_NET_CONFIG_SETTINGS=y
+CONFIG_TEST_RANDOM_GENERATOR=y
 CONFIG_NET_CONFIG_MY_IPV4_ADDR="192.168.1.1"
 CONFIG_NET_CONFIG_MY_IPV6_ADDR="2001:db8::1"