mkosi: Make sure initrd coredumps are also stored in the journal

Let's also translate the coredump config into a dropin so we don't
accidentally override any existing configuration.

mkosi.images/system/initrd/mkosi.conf

@@ -2,4 +2,6 @@
 
 [Content]
 PostInstallationScripts=../mkosi.sanitizers.chroot
-ExtraTrees=../leak-sanitizer-suppressions:/usr/lib/systemd/leak-sanitizer-suppressions
+ExtraTrees=
+    ../leak-sanitizer-suppressions:/usr/lib/systemd/leak-sanitizer-suppressions
+    ../coredump-journal-storage.conf:/usr/lib/systemd/coredump.conf.d/10-coredump-journal-storage.conf

mkosi.images/system/mkosi.conf

@@ -12,6 +12,7 @@ Autologin=yes
 ExtraTrees=
         %D/mkosi.crt:/usr/lib/verity.d/mkosi.crt # sysext verification key
         leak-sanitizer-suppressions:/usr/lib/systemd/leak-sanitizer-suppressions
+        coredump-journal-storage.conf:/usr/lib/systemd/coredump.conf.d/10-coredump-journal-storage.conf
 
 PostInstallationScripts=mkosi.sanitizers.chroot