esp: Check in sources for flasher stub

Recovered from https://github.com/cesanta/mongoose-os/tree/f626b5113861634e9a4acc96bdb73de29ed8a4d0/common/platforms/
refactored the build system.

Example to rebuild and test esp8266 stub (read flash):
```
  $ make stubs mos && ./mos flash-read --platform esp8266 --esp-baud-rate=1500000 test.bin --v 4 --logtostderr
```

Signed-off-by: Deomid "rojer" Ryabkov <rojer@rojer.me>

Makefile

@@ -1,5 +1,5 @@
 .DEFAULT_GOAL := mos
-.PHONY: all build clean clean-tools clean-version deploy-fwbuild deploy-mos-binary deps downloads fwbuild-instance fwbuild-manager generate install linux mac mos version win
+.PHONY: all build clean clean-tools clean-version deploy-fwbuild deploy-mos-binary deps downloads fwbuild-instance fwbuild-manager generate genstubs install linux mac mos stubs version win
 
 TAG ?= latest
 
@@ -57,6 +57,15 @@ vendor/modules.txt:
 
 deps: vendor/modules.txt
 
+genstubs:
+	cli/flash/esp32/stub/genstub.sh
+	cli/flash/esp8266/stub/genstub.sh
+
+# Build the flasher stubs
+# E.g.:
+#   make stubs mos && ./mos flash-read --platform esp8266 --esp-baud-rate=1500000 test.bin --v 4 --logtostderr
+stubs: genstubs generate
+
 generate: $(GOBIN)/go-bindata $(GOBIN)/go-bindata-assetfs
 	$(GO) generate \
 	  github.com/mongoose-os/mos/cli/... \

cli/flash/esp/flasher/flasher_client.go

@@ -91,9 +91,9 @@ func (fc *FlasherClient) connect(romBaudRate, baudRate uint) error {
 	var stubJSON []byte
 	switch fc.ct {
 	case esp.ChipESP8266:
-		stubJSON = esp8266.MustAsset("data/stub_flasher.json")
+		stubJSON = esp8266.MustAsset("stub/stub.json")
 	case esp.ChipESP32:
-		stubJSON = esp32.MustAsset("data/stub_flasher.json")
+		stubJSON = esp32.MustAsset("stub/stub.json")
 	default:
 		return errors.Errorf("unknown chip type %d", fc.ct)
 	}