pkg/tinydtls: Set buffer size as required for gcoap

pkg/tinydtls/Makefile.include

@@ -69,3 +69,18 @@ HANDSHAKE_MAX := $(or $(CONFIG_DTLS_HANDSHAKE_MAX),$(patsubst -DCONFIG_DTLS_HAND
 ifneq (,$(HANDSHAKE_MAX))
     CFLAGS += -DDTLS_HANDSHAKE_MAX=$(HANDSHAKE_MAX)
 endif
+
+ifneq $(filter gcoap,$(USEMODULE))
+    # Configuring the buffer large enough that a full Gcoap packet can be
+    # encrypted or decrypted.
+
+    # This is the default in gcoap.h, which we don't have access to, so it is copied over.
+    CONFIG_GCOAP_PDU_BUF_SIZE := $(or $(CONFIG_GCOAP_PDU_BUF_SIZE),128)
+
+    # If there were another way to set up DTLS_MAX_BUF, we'd need to set the
+    # maximum of these here.
+    #
+    # 29 bytes are the overhead measured with Wireshark on packets exchanged in
+    # default configuration; adding some to be safe against variable size fields.
+    CFLAGS += "-DDTLS_MAX_BUF=($(CONFIG_GCOAP_PDU_BUF_SIZE) + 36)"
+endif