Add Python package required for image signing

Add 'imgtool' Python package, which is used to sign
the firmware images to be bootable by MCUBoot.

Change-Id: I7e63ed5c91de09e0516b01bfcc1c7abda480684f
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/xenial-amd64-tf-m-build/requirements_python3.txt b/xenial-amd64-tf-m-build/requirements_python3.txt
index 3704eb0..51d3cd3 100644
--- a/xenial-amd64-tf-m-build/requirements_python3.txt
+++ b/xenial-amd64-tf-m-build/requirements_python3.txt
@@ -10,3 +10,4 @@
 sphinx-rtd-theme==0.4.3
 sphinxcontrib-plantuml==0.15
 xmltodict==0.12.0
+imgtool==1.6.0