[nrf noup] net: tests: crypto: Adding legacy Crypto support ipv6 tests

-This adds crypto support for ipv6 tests by enabling
 CONFIG_MBEDTLS_LEGACY_CRYPTO_C

Signed-off-by: Frank Audun Kvamtrø <[email protected]>
(cherry picked from commit 2553081)

Author: frkv

tests/net/ipv6/prj.conf

@@ -33,6 +33,7 @@ CONFIG_NET_IF_MAX_IPV6_COUNT=2
 CONFIG_NET_IPV6_PE=y
 CONFIG_NET_IPV6_PE_FILTER_PREFIX_COUNT=2
 CONFIG_NET_IPV6_PE_PREFER_PUBLIC_ADDRESSES=n
+CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y
 
 # Increase the stack a bit for mps2/an385
 CONFIG_NET_RX_STACK_SIZE=1700