imgtool: Use a requirements file for dependencies

Instead of relying on distro packages standardize the imgtool
dependencies into a requirements.txt.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

docs/imgtool.md

@@ -6,13 +6,9 @@ this script should be preferred to the manual steps described in
 `doc/signed_images.md`.
 
 This program is written for Python3, and has several dependencies on
-Python libraries.  These can be installed using 'pip3' manually:
+Python libraries.  These can be installed using 'pip3':
 
-    pip3 install --user cryptography
-
-or, on Ubuntu, using the package manager:
-
-    sudo apt-get install python3-cryptography
+    pip3 install --user -r scripts/requirements.txt
 
 ## Managing keys
 

scripts/requirements.txt

@@ -0,0 +1 @@
+cryptography