image: add functions to add and remove signatures

Rather than setting ->sigbuf directly, add two functions to handle image
signature addition and removal:

 image_add_signature(image, sig, sigsize);
 image_remove_signature(image);

And warn when a signature is to be overwritten.

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

tests/Makefile.am

@@ -45,7 +45,9 @@ TESTS = sign-verify.sh \
 	verify-missing-image.sh \
 	verify-missing-cert.sh \
 	sign-invalidattach-verify.sh \
-	cert-table-header.sh
+	cert-table-header.sh \
+	resign-warning.sh \
+	reattach-warning.sh
 
 EXTRA_DIST = $(test_lds) test.S $(TESTS) $(check_SCRIPTS)
 CLEANFILES = $(test_key) $(test_cert) $(test_image)

tests/reattach-warning.sh

@@ -0,0 +1,12 @@
+#!/bin/bash -e
+
+. "$srcdir/common.sh"
+
+signed="test.signed"
+sig="test.sig"
+
+"$sbsign" --cert "$cert" --key "$key" --detached --output "$sig" "$image"
+cp "$image" "$signed"
+"$sbattach" --attach "$sig" "$signed"
+"$sbattach" --attach "$sig" "$signed" 2>&1 |
+	grep '^warning: overwriting'

tests/resign-warning.sh

@@ -0,0 +1,9 @@
+#!/bin/bash -e
+
+. "$srcdir/common.sh"
+
+signed="test.signed"
+
+"$sbsign" --cert "$cert" --key "$key" --output "$signed" "$image"
+"$sbsign" --cert "$cert" --key "$key" --output "$signed" "$signed" 2>&1 |
+	grep '^warning: overwriting'