pkg/nanopb: fix for proto files with includes

Proto files can include other proto files.
E.g. nanopb comes with a `nanopb.proto` include that contains some
nanopb-specific extensions.

This patch makes it possible to include this file as well as specifying
additional proto include folders.

I added the include to `simple.proto` so it gets exercised by the test.

pkg/nanopb/Makefile.gensrc

@@ -2,13 +2,20 @@ PROTOC ?= protoc
 PROTOC_GEN_NANOPB ?= $(PKGDIRBASE)/nanopb/generator/protoc-gen-nanopb
 
 PROTOBUF_FILES ?= $(wildcard *.proto)
+PROTOBUF_PATH  ?= $(CURDIR)
 GENSRC   += $(PROTOBUF_FILES:%.proto=$(BINDIR)/$(MODULE)/%.pb.c)
 GENOBJC  := $(GENSRC:%.c=%.o)
 
-ifneq (, $(PROTOBUF_FILES))
+ifneq (,$(PROTOBUF_FILES))
   INCLUDES += -I$(BINDIR)/$(MODULE)
 endif
 
+# workaround for old protoc
+PROTO_INCLUDES += -I.
+# add nanopb specific includes
+PROTO_INCLUDES += -I$(PKGDIRBASE)/nanopb/generator/proto
+PROTO_INCLUDES += -I$(PROTOBUF_PATH)
+
 $(SRC): $(GENSRC)
 
 $(GENSRC): $(PROTOBUF_FILES)
@@ -18,6 +25,6 @@ $(GENSRC): $(PROTOBUF_FILES)
 		make -C $(PKGDIRBASE)/nanopb/generator/proto && \
 		for protofile in $(PROTOBUF_FILES); do \
 			protoc --plugin=protoc-gen-nanopb=$(PROTOC_GEN_NANOPB) \
-				--nanopb_out="$$D" \
+				--nanopb_out="$$D" $(PROTO_INCLUDES) \
 				$^ \
 				; done

tests/pkg_nanopb/simple.proto

@@ -2,6 +2,7 @@
 // one message.
 
 syntax = "proto2";
+import "nanopb.proto";
 
 message SimpleMessage {
     required int32 lucky_number = 1;