Fix build errors for IoT-Lab tutorial

[pipex]

Following the Contiki-NG IoT-Lab tutorial fails with the following build errors

• Duplicate 'const' declaration in unique_id.h

  In file included from ../../../arch/platform/iotlab/openlab/platform/iotlab-m3/iotlab-m3.c:34:0:
  ../../../arch/platform/iotlab/openlab/drivers/unique_id.h:60:28: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
  extern const openlab_uid_t const *uid;

• Duplicate 'const' declaration in unique_id.c

  ../../../arch/platform/iotlab/openlab/drivers/stm32/unique_id.c:32:21: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
  const openlab_uid_t const *uid = (const openlab_uid_t *const) UNIQUE_ID_BASE_ADDRESS;

• MAC_CONF_WITH_TSCH undeclared

  ../../../arch/platform/iotlab/./radio-rf2xx.c: In function 'rf2xx_wr_transmit':
  ../../../arch/platform/iotlab/./radio-rf2xx.c:73:25: error: 'MAC_CONF_WITH_TSCH' undeclared (first use in this function); did you mean 'MAC_CONF_WITH_CSMA'?
  #define RF2XX_WITH_TSCH MAC_CONF_WITH_TSCH

This PR fixes the build errors.