scripts: Allow image version number

Add an --image-version argument to zep2newt.py to allow the major
version number to be set.  This helps make testing easier by having
differing version numbers.
diff --git a/sign.sh b/sign.sh
index 269032d..6fe08f5 100755
--- a/sign.sh
+++ b/sign.sh
@@ -9,6 +9,7 @@
     --out shell.signed.bin \
     --vtoff 0x200 \
     --word-size 8 \
+    --image-version 3 \
     --bit --pad 0x20000
 
 ./scripts/zep2newt.py \
@@ -17,4 +18,5 @@
     --sig RSA \
     --vtoff 0x200 \
     --word-size 8 \
+    --image-version 2 \
     --out hello.signed.bin