image: Use size of image data when writing images

When detaching a signature, we need to know the size of the
non-signature data. So, add a data_size member to struct image, and
populate it when we iterate through the section table.

When writing the image, use data_size rather than size, so we don't
unnecessarily add the (now unused) signature data.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>

tests/detach-remove.sh

@@ -0,0 +1,15 @@
+#!/bin/bash -ex
+
+signed="test.signed"
+unsigned="test.unsigned"
+
+"$sbsign" --cert "$cert" --key "$key" --output "$signed" "$image"
+cp "$signed" "$unsigned"
+"$sbattach" --remove "$unsigned"
+
+# ensure that there is no security directory
+objdump -p $unsigned | grep -q '0\+ 0\+ Security Directory'
+
+# ensure that the unsigned file is the same size as our original binary
+[ $(stat --format=%s "$image") -eq $(stat --format=%s "$unsigned") ]
+