kernel-install: support full set of config files and drop-ins

This brings the handling of config for kernel-install in line with most of
systemd, i.e. we search the set of paths for the main config file, and the full
set of drop-in paths for drop-ins.

This mirrors what 07f5e35fe7 did for udev.conf.
That change worked out fine, so I hope this one will too.

The update in the man page is minimal. I think we should split out a separate
page for the config file later on.

One motivating use case is to allow a drop-in to be created for temporary
config overrides and then removed after the operation is done.

src/kernel-install/test-kernel-install.sh

@@ -125,7 +125,8 @@ grep -qE 'initrd' "$BOOT_ROOT/the-token/1.1.1/initrd"
 
 # Install UKI
 if [ -f "$ukify" ]; then
-    cat >>"$D/sources/install.conf" <<EOF
+    mkdir "$D/sources/install.conf.d"
+    cat >>"$D/sources/install.conf.d/override.conf" <<EOF
 layout=uki
 uki_generator=ukify
 EOF
@@ -146,6 +147,8 @@ EOF
     "$ukify" inspect "$uki" | grep -qE '^.initrd'
     "$ukify" inspect "$uki" | grep -qE '^.linux'
     "$ukify" inspect "$uki" | grep -qE '^.dtb'
+
+    rm "$D/sources/install.conf.d/override.conf"
 fi
 
 # Test bootctl