imgtool: Add support for Intel Hex images

Some platforms prefer images in the Intel Hex (.hex) format. Implement
support for signing images in this format in imgtool.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/scripts/requirements.txt b/scripts/requirements.txt
index 0d38bc5..1198d45 100644
--- a/scripts/requirements.txt
+++ b/scripts/requirements.txt
@@ -1 +1,2 @@
 cryptography
+intelhex