littlefs-do binary to work with spi raw file (#52)

Add helper to modify spi raw file, to make experimenting with it easier.

```sh
$ ./littlefs-do --help
Usage: ./littlefs-do <command> [options]
Commands:
  -h, --help           show this help message for the selected command and exit
  -v, --verbose        print status messages to the console
  stat                 show information of specified file or directory
  ls                   list available files in 'spiNorFlash.raw' file
  mkdir                create directory
  rmdir                remove directory
  rm                   remove directory or file
  cp                   copy files into or out of flash file
  settings             list settings from 'settings.h'
```

In the process restructure the CMake file for less duplicate
includes/defines for both executables (`infinisim` and `littlefs-do`).

Upload the `littlefs-do` binary built by the CI additionally to the `infinisim` binary.
Use the updated upload-artifact@v3 template to do that.

README.md

@@ -115,6 +115,26 @@ Using the keyboard the following events can be triggered:
 - `i` ... take screenshot
 - `I` ... start/stop Gif scren capture
 
+## Littlefs-do helper
+
+To help working with the SPI-raw file the tool `littlefs-do` is provided.
+The SPI-raw file emulates the persistent 4MB storage available over the SPI bus on the PineTime.
+
+```sh
+$ ./littlefs-do --help
+Usage: ./littlefs-do <command> [options]
+Commands:
+  -h, --help           show this help message for the selected command and exit
+  -v, --verbose        print status messages to the console
+  stat                 show information of specified file or directory
+  ls                   list available files in 'spiNorFlash.raw' file
+  mkdir                create directory
+  rmdir                remove directory
+  rm                   remove directory or file
+  cp                   copy files into or out of flash file
+  settings             list settings from 'settings.h'
+```
+
 ## Licenses
 
 This project is released under the GNU General Public License version 3 or, at your option, any later version.