Update Thread PSKd default value

Thread PSKd can contain only uppercase characters.

Author: Arto Kinnunen

features/nanostack/FEATURE_NANOSTACK/mbed-mesh-api/mbed_lib.json

@@ -12,7 +12,7 @@
         "6lowpan-nd-psk-key": "{0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf}",
         "6lowpan-nd-sec-level": 5,
         "6lowpan-nd-device-type": "NET_6LOWPAN_ROUTER",
-        "thread-pskd": "\"abcdefghijklmno\"",
+        "thread-pskd": "\"ABCDEFGH\"",
         "thread-use-static-link-config": true,
         "thread-config-channel-mask": "0x7fff800",
         "thread-config-channel-page": 0,