build_image.py: map vexpress-debug to vexpress hwpack

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
diff --git a/build_images.py b/build_images.py
index 2430a7c..76a90dc 100755
--- a/build_images.py
+++ b/build_images.py
@@ -77,6 +77,7 @@
     'snowball': HwPack('snowball_sd', True),
     'ti-panda-x11-base': HwPack('panda'),
     'vexpress': HwPack('vexpress'),
+    'vexpress-debug': HwPack('vexpress'),
     'vexpress64': HwPack('vexpress'),
 }
 # a mapping of binary image to its image_file size