tests/gnrc_tcp/ : Set 'CONFIG_' if not Kconfig

Set CONFIG_GNRC_TCP_MSL and
CONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION if not set via Kconfig

tests/gnrc_tcp/Makefile

@@ -13,8 +13,6 @@ TIMEOUT_US ?= 3000000
 TEST_ON_CI_BLACKLIST += all
 
 CFLAGS += -DSHELL_NO_ECHO
-CFLAGS += -DCONFIG_GNRC_TCP_MSL=$(MSL_US)
-CFLAGS += -DCONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION=$(TIMEOUT_US)
 CFLAGS += -DGNRC_NETIF_SINGLE           # Only one interface used and it makes
                                         # shell commands easier
 
@@ -45,3 +43,14 @@ ethos:
 	$(Q)env -u CC -u CFLAGS make -C $(RIOTTOOLS)/ethos
 
 include $(RIOTBASE)/Makefile.include
+
+# Set CONFIG_GNRC_TCP_MSL via CFLAGS if not being set via Kconfig
+ifndef CONFIG_GNRC_TCP_MSL
+  CFLAGS += -DCONFIG_GNRC_TCP_MSL=$(MSL_US)
+endif
+
+# Set CONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION via CFLAGS if not being set
+# via Kconfig
+ifndef CONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION
+  CFLAGS += -DCONFIG_GNRC_TCP_CONNECTION_TIMEOUT_DURATION=$(TIMEOUT_US)
+endif