Add new DFU string index to show device state

[keeramis]

Problem

Currently, there is no way to distinguish between Open and Protected Devices in Service Mode when the device is in DFU mode by checking the DFU segments

Solution

Add a new string index which can be queried by reading the string descriptor that gives the device state

Steps to Test

1. Build and flash this bootloader
2. Use particle-usb to test this. Accurate device state detection for devices in DFU particle-usb#115

Example App

void setup() {
  /* A minimal example app is super helpful 
   * for testing new features and fixes. 
   * A link to a Docs PR is even better!
   */
}

void loop() {

}

References

Links to the Community, Docs, Other Issues, etc..

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

@keeramis Looks like this is failing on Gen 4 platforms:

make -s clean all PLATFORM_ID=32 COMPILE_LTO=n DEBUG_BUILD=n MODULAR=y USE_SWD_JTAG=n USE_SWD=n APP=tinker
  /firmware/modules/tron/system-part1/makefile /firmware/modules/tron/user-part/makefile
./src/rtl872x/usbd_device.cpp: In function 'char* {anonymous}::device_state_as_string(char*)':
./src/rtl872x/usbd_device.cpp:53:16: error: 'security_mode_get' was not declared in this scope
   53 |     int mode = security_mode_get(nullptr);
      |                ^~~~~~~~~~~~~~~~~
./src/rtl872x/usbd_device.cpp:56:19: error: 'security_mode_is_overridden' was not declared in this scope
   56 |       memcpy(buf, security_mode_is_overridden() ? "sm=s" : "sm=o", 4);
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~

[keeramis]

Fixed the error. It was missing the header file. I tested on my Argon and my P2.