[nrfconnect] Added calculating unique id length based on kconfig

Currently the unique id length is fixed to 16 B and in nrfconnect
platform we allow to set unique id value through kconfig, which may
lead to a problem that length of selected data will not match the
unique id length hardcoded to 16 B.

src/platform/nrfconnect/CHIPDevicePlatformConfig.h

@@ -249,3 +249,7 @@
 #ifdef CONFIG_CHIP_EXTENDED_DISCOVERY
 #define CHIP_DEVICE_CONFIG_ENABLE_EXTENDED_DISCOVERY 1
 #endif // CONFIG_CHIP_EXTENDED_DISCOVERY
+
+#ifndef CHIP_DEVICE_CONFIG_ROTATING_DEVICE_ID_UNIQUE_ID_LENGTH
+#define CHIP_DEVICE_CONFIG_ROTATING_DEVICE_ID_UNIQUE_ID_LENGTH ((sizeof(CONFIG_CHIP_DEVICE_ROTATING_DEVICE_UID) - 1) / 2)
+#endif // CHIP_DEVICE_CONFIG_ROTATING_DEVICE_ID_UNIQUE_ID_LENGTH