net: disable MBEDTLS in SPL

Building SPL fails with MBEDTLS enabled.
Currently we don't need it there.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/lib/lwip/Makefile b/lib/lwip/Makefile
index 19e5c68..fe2b64c 100644
--- a/lib/lwip/Makefile
+++ b/lib/lwip/Makefile
@@ -54,5 +54,5 @@
 	lwip/src/core/udp.o \
 	lwip/src/netif/ethernet.o
 
-obj-$(CONFIG_MBEDTLS_LIB_TLS) += lwip/src/apps/altcp_tls/altcp_tls_mbedtls.o \
+obj-$(CONFIG_$(XPL_)MBEDTLS_LIB_TLS) += lwip/src/apps/altcp_tls/altcp_tls_mbedtls.o \
 	lwip/src/apps/altcp_tls/altcp_tls_mbedtls_mem.o