doc: Added serial recovery documentation

- [Encrypted images](encrypted_images.md)

- [imgtool](imgtool.md) - image signing and key management

- [ECDSA](ecdsa.md) - information about ECDSA signature formats

- [Serial Recovery](serial_recovery.md) - MCUmgr as the serial recovery protocol

- Usage instructions:

  - [Zephyr](readme-zephyr.md)

  - [Apache Mynewt](readme-mynewt.md)

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# Serial recovery

MCUboot implements a Simple Management Protocol (SMP) server.

SMP is a basic transfer encoding for use with the MCUmgr management protocol. 

See the Zephyr [Device Management](https://docs.zephyrproject.org/latest/services/device_mgmt/index.html#device-mgmt) documentation for more information about MCUmgr and SMP.

MCUboot supports the following subset of the MCUmgr commands:

* echo (OS group)

* reset (OS group)

* image list (IMG group)

* image upload (IMG group)

It can also support system-specific MCUmgr commands depending on the given mcuboot-port

if the ``MCUBOOT_PERUSER_MGMT_GROUP_ENABLED`` option is enabled.

The serial recovery feature can use any serial interface provided by the platform.

## Image uploading

Uploading an image is targeted to the primary slot by default.

An image can be loaded to other slots only when the ``MCUBOOT_SERIAL_DIRECT_IMAGE_UPLOAD`` option is enabled for the platform.

MCUboot supports progressive erasing of a slot to which an image is uploaded to if the ``MCUBOOT_ERASE_PROGRESSIVELY`` option is enabled.

As a result, a device can receive images smoothly, and can erase required part of a flash automatically.

## Configuration of serial recovery

How to enable and configure the serial recovery feature depends on the given mcuboot-port implementation.

Refer to the respective documentation and source code for more details.

## Entering serial recovery mode

Entering the serial recovery mode is usually possible right after a device reset, for instance as a reaction on a GPIO pin state.

Refer to the given mcuboot-port details to get information on how to enter the serial recovery mode.

## Serial recovery mode usage

### MCU Manager CLI installation

The MCUmgr command line tool can be used as an SMP client for evaluation purposes.

The tool is available under the [MCU Manager](https://github.com/apache/mynewt-mcumgr-cli)

Github page and is described in the Zephyr 

[MCU Manager CLI](https://docs.zephyrproject.org/latest/services/device_mgmt/mcumgr.html#mcumgr-cli) documentation.

Use the following command to install the MCU Manager CLI tool:

``` console

go install github.com/apache/mynewt-mcumgr-cli/mcumgr@latest

```

Enter serial recovery mode in the device and use an SMP client application for communication with the MCUboot's SMP server.

### Connection configuration

Use the following command to set the connection configuration,

for linux:

``` console

mcumgr conn add serial_1 type="serial" connstring="dev=/dev/ttyACM0,baud=115200"

```

for windows:

``` console

mcumgr conn add serial_1 type="serial" connstring="COM1,baud=115200"

```

### Image management

The connection configuration must be established to perform the following image-related commands:

* Upload the image:

  ``` console

  mcumgr image upload <path-to-signed-image-bin-file> -c serial_1

  ```

  Once done successfully, the following notification will be displayed:

  ``` console

  20.25 KiB / 20.25 KiB [=================================] 100.00% 3.08 KiB/s 6s

  Done

  ```

* List all images:

  ``` console

  mcumgr image list -c serial_1

  ```

  The terminal will show the response from the module:

  ``` console

  Images:

   image=0 slot=0

      version: 0.0.0.0

      bootable: false

      flags: 

      hash: Unavailable

  Split status: N/A (0)

  ```

### Device reset

Reset your device with the following command:

``` console

mcumgr reset -c serial_1

```

The terminal should respond:

``` console

Done

```