Split mbedtls_gcm_init() -> gcm_setkey()

ChangeLog

@@ -16,6 +16,7 @@ API Changes
    * The following _init() functions that could return errors have
      been split into an _init() that returns void and another function:
      mbedtls_ccm_init() -> mbedtls_ccm_setkey()
+     mbedtls_gcm_init() -> mbedtls_gcm_setkey()
    * In the threading layer, mbedtls_mutex_init() and mbedtls_mutex_free() now
      return void.
    * ecdsa_write_signature() gained an addtional md_alg argument and