Add python packages click and cryptography to build instructions

The script `tools/mcuboot/imgtool/main.py` imports the python packages
`click` and `cryptography` to create the target `pinetime-mcuboot-app`.
Add it tothe build instructions, as it was not installed on my system.

6f9f0e8b0e/tools/mcuboot/imgtool/main.py (L19)
6f9f0e8b0e/tools/mcuboot/imgtool/keys/__init__.py (L19)

Also add the commands to install the python requirements into a python
venv virtual environment.

doc/buildAndProgram.md

@@ -3,7 +3,14 @@
 To build this project, you'll need:
  - A cross-compiler : [ARM-GCC (9-2020-q2-update)](https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads/9-2020-q2-update)
  - The NRF52 SDK 15.3.0 : [nRF-SDK v15.3.0](https://developer.nordicsemi.com/nRF5_SDK/nRF5_SDK_v15.x.x/nRF5_SDK_15.3.0_59ac345.zip)
- - The `cbor` and `intelhex` modules for Python 3
+ - The Python 3 modules `cbor`, `intelhex`, `click` and `cryptography` modules for the `mcuboot` tool (see [requirements.txt](../tools/mcuboot/requirements.txt))
+   - To to keep the system clean a python virtual environment (`venv`) can be used to install the python modules into
+     ```sh
+	 python -m venv .venv
+	 source .venv/bin/activate
+	 python -m pip install wheel
+	 python -m pip install -r tools/mcuboot/requirements.txt
+	 ```
  - A reasonably recent version of CMake (I use 3.16.5)
 
 ## Build steps