kernel-install/90-loaderentry: do not read dtbs from /boot

/boot is not trusted, so we shouldn't use load files from there. Also, space in
/boot is limited, so it doesn't make sense to install the files under one
location there and then copy them to a different location. We should only copy
the files from /usr somewhere and then install it in the appropriate place under
/boot.

Also use "/usr/lib" instead of the "/lib" prefix. We don't support unmerged-user
anymore.

Addresses some of the feedback in
https://github.com/systemd/systemd/pull/28582#discussion_r1285820556.

src/kernel-install/90-loaderentry.install.in

@@ -126,11 +126,9 @@ elif [ -f /usr/lib/kernel/devicetree ]; then
 fi
 if [ -n "$DEVICETREE" ]; then
     for prefix in \
-        "/boot/dtb-$KERNEL_VERSION" \
-        "/boot/dtbs/$KERNEL_VERSION" \
-        "/lib/firmware/$KERNEL_VERSION/device-tree" \
-        "/lib/linux-image-$KERNEL_VERSION" \
-        "/lib/modules/$KERNEL_VERSION/dtb"
+        "/usr/lib/firmware/$KERNEL_VERSION/device-tree" \
+        "/usr/lib/linux-image-$KERNEL_VERSION" \
+        "/usr/lib/modules/$KERNEL_VERSION/dtb"
     do
         [ -f "$prefix/$DEVICETREE" ] || continue
         DEVICETREE_SRC="$prefix/$DEVICETREE"