sbattach: fix --detach

sbattach --detach isn't working, as we're not properly setting sigbuf in
image_pecoff parse.

This change ensures we populate sigbuf when we find a valid cert table.
Also, add a test case for this.

Bug report & initial patch from from Steve Langasek.

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

tests/Makefile.am

@@ -37,6 +37,7 @@ $(test_cert): $(test_key) Makefile
 
 TESTS = sign-verify.sh \
 	sign-verify-detached.sh \
+	sign-detach-verify.sh \
 	sign-attach-verify.sh \
 	sign-missing-image.sh \
 	sign-missing-cert.sh \

tests/sign-detach-verify.sh

@@ -0,0 +1,10 @@
+#!/bin/bash -e
+
+. "$srcdir/common.sh"
+
+signed="test.signed"
+sig="test.sig"
+
+"$sbsign" --cert "$cert" --key "$key" --output "$signed" "$image"
+"$sbattach" --detach "$sig" "$signed"
+"$sbverify" --cert "$cert" --detached $sig "$image"