Use shim 15.7 to support Secure Boot

Use shim-signed and shim-helpers-{arch}-signed from Debian:
https://packages.debian.org/bookworm/shim-signed

shim-signed/COPYRIGHT

@@ -0,0 +1,55 @@
+Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: shim
+Upstream-Contact: Peter Jones <pjones@redhat.com>
+Source: https://github.com/rhboot/shim
+
+Files: *
+Copyright: 2012 Red Hat, Inc
+	2009-2012 Intel Corporation
+License: BSD-2-Clause
+
+Files: debian/po/cs.po
+Copyright: 2018 Michal Simunek <michal.simunek@gmail.com>
+License: BSD-2-Clause
+
+Files: debian/po/de.po
+Copyright: 2017, 2018 Markus Hiereth <markus.hiereth@freenet.de>
+License: BSD-2-Clause
+
+Files: debian/po/fr.po
+Copyright: 2017, 2018 Alban Vidal <alban.vidal@zordhak.fr>
+License: BSD-2-Clause
+
+Files: debian/po/nl.po
+Copyright: 2017, 2018 Frans Spiesschaert <Frans.Spiesschaert@yucom.be>
+License: BSD-2-Clause
+
+Files: debian/po/pt.po
+Copyright: 2017, 2018 Rui Branco <ruipb@debianpt.org>
+License: BSD-2-Clause
+
+License: BSD-2-Clause
+ Redistribution and use in source and binary forms, with or without
+ modification, are permitted provided that the following conditions
+ are met:
+ .
+ Redistributions of source code must retain the above copyright
+ notice, this list of conditions and the following disclaimer.
+ .
+ Redistributions in binary form must reproduce the above copyright
+ notice, this list of conditions and the following disclaimer in the
+ documentation and/or other materials provided with the
+ distribution.
+ .
+ THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ OF THE POSSIBILITY OF SUCH DAMAGE.