Rename macro SETUP_ALT to SETUP_TEARDOWN_ALT

Rename the macro MBEDTLS_PLATFORM_SETUP_ALT to
MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT to make the name more descriptive
as this macro enables/disables both functions.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index ffeeb34..de99938 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -163,7 +163,7 @@
 //#define MBEDTLS_PLATFORM_PRINTF_ALT
 //#define MBEDTLS_PLATFORM_SNPRINTF_ALT
 //#define MBEDTLS_PLATFORM_NV_SEED_ALT
-//#define MBEDTLS_PLATFORM_SETUP_ALT
+//#define MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT
 
 /**
  * \def MBEDTLS_DEPRECATED_WARNING