96b_nitrogen.conf: Introduce board config

Add board config for 96b_nitrogen.
The config will select IPV6 instead of IPV4,
disable unused options and add configuration options
for IP addresses to override defaults.

Note: IPV6 addresses need to be specified by the user.

Signed-off-by: Zbigniew Bodek <zbigniew.bodek@huawei.com>

boards/96b_nitrogen.conf

+# Networking Config:
+CONFIG_NET_IPV4=n
+CONFIG_NET_IPV6=y
+
+# Disable unneeded settings from the base prj.conf:
+CONFIG_TEST_RANDOM_GENERATOR=n
+CONFIG_DNS_RESOLVER=n
+CONFIG_NET_CONFIG_MY_IPV4_ADDR=""
+CONFIG_NET_CONFIG_PEER_IPV4_ADDR=""
+
+# Specify networking addresses
+CONFIG_NET_CONFIG_MY_IPV6_ADDR=""
+CONFIG_NET_CONFIG_PEER_IPV6_ADDR=""