Add SPI flash VFS device driver

PUBLISHED_FROM=13ceb95aca330874537e37a250909875c24f46cc

Author: Deomid Ryabkov

mos.yml

@@ -17,6 +17,7 @@ libs:
   - origin: https://github.com/mongoose-os-libs/rpc-service-i2c
   - origin: https://github.com/mongoose-os-libs/rpc-service-ota
   - origin: https://github.com/mongoose-os-libs/rpc-uart
+  - origin: https://github.com/mongoose-os-libs/vfs-dev-spi-flash
 
     # libs necessary for the current app
   - origin: https://github.com/mongoose-os-libs/arduino-adafruit-ssd1306