mkosi: Use mkosi.initrd.conf

A more straightforward way to extend the default initrd rather than
using a subimage. This mainly allows us to take advantage of mkosi
automatically figuring out whether the initrd needs to be built or not,
speeding up builds when building directory images as the initrd can be
skipped in that case.

mkosi/mkosi.conf

@@ -3,7 +3,6 @@
 [Config]
 MinimumVersion=commit:d6ddc66acb05d3d761d3f600c6b029319db9ae95
 Dependencies=
-        initrd
         minimal-base
         minimal-0
         minimal-1
@@ -58,7 +57,6 @@ ExtraTrees=
         %O/minimal-base:/usr/share/TEST-13-NSPAWN-container-template
         %O/initrd:/exitrd
 
-Initrds=%O/initrd
 KernelInitrdModules=default
 
 # Disable relabeling by default as it only matters for TEST-06-SELINUX, takes a non-trivial amount of time

mkosi/mkosi.initrd.conf/mkosi.conf

@@ -2,7 +2,6 @@
 
 [Include]
 Include=
-        mkosi-initrd
         %D/mkosi/mkosi.sanitizers
         %D/mkosi/mkosi.coverage