Enable zperf on Genesys (#16)

- These setting should migrate to board level defconfig
- Have seen 113 Mbps even with NEWLIBC enabled and disabled. Unclear why sometimes seeing 1 Mbps

To build: west build -b genesys2 samples/net/zperf -- -DOVERLAY_CONFIG=overlay-lowrisc.conf

Usage:
On linux running iperf 2.0.5:  iperf -s
On zperf app: zperf tcp upload 192.168.1.9

Author: brudley

samples/net/zperf/overlay-lowrisc.conf

@@ -0,0 +1,28 @@
+
+
+# General config
+#
+CONFIG_NEWLIB_LIBC=y
+CONFIG_MAIN_STACK_SIZE=4096
+
+# Networking config
+CONFIG_NETWORKING=y
+CONFIG_NET_IPV4=y
+CONFIG_NET_IPV6=n
+CONFIG_NET_TCP=y
+CONFIG_NET_SOCKETS=y
+CONFIG_NET_SOCKETS_POSIX_NAMES=y
+
+# Network driver config
+CONFIG_TEST_RANDOM_GENERATOR=y
+
+# Network address config
+CONFIG_NET_CONFIG_SETTINGS=y
+CONFIG_NET_CONFIG_NEED_IPV4=y
+CONFIG_NET_CONFIG_MY_IPV4_ADDR="192.168.1.78"
+CONFIG_NET_L2_ETHERNET=y
+CONFIG_ETH_LOWRISC=y
+
+#Logging thread gets hung up wthout this
+CONFIG_LOG_IMMEDIATE=y
+