mkosi: install EFI packages only on EFI architectures

sbsigntool, systemd-boot and systemd-boot-efi do not  exist on other
architectures

mkosi.images/system/mkosi.conf.d/10-debian-ubuntu/mkosi.conf

@@ -20,8 +20,6 @@ VolatilePackages=
         libsystemd-dev
         libudev-dev
         systemd
-        systemd-boot
-        systemd-boot-efi
         systemd-container
         systemd-coredump
         systemd-dev
@@ -74,7 +72,6 @@ Packages=
         python3-pexpect
         python3-psutil
         quota
-        sbsigntool
         softhsm2
         squashfs-tools
         stress

mkosi.images/system/mkosi.conf.d/10-debian-ubuntu/mkosi.conf.d/efi.conf

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# sbsigntool exists only on UEFI architectures
+
+[Match]
+Architecture=|x86
+Architecture=|x86-64
+Architecture=|arm
+Architecture=|arm64
+Architecture=|riscv32
+Architecture=|riscv64
+
+[Content]
+Packages=
+        sbsigntool
+        systemd-boot
+        systemd-boot-efi