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>
diff --git a/scripts/requirements.txt b/scripts/requirements.txt
new file mode 100644
index 0000000..0d38bc5
--- /dev/null
+++ b/scripts/requirements.txt
@@ -0,0 +1 @@
+cryptography