base-files: don't overwrite model name set by target

The condition is always true due to the literal string followed the
-n test parameter. A model name set by target scripts always gets
overwritten this way.

Change the condition to check for an already existing destination file
as it was before 5e85ae9e ("base-files: fix error message during boot").

Signed-off-by: Mathias Kresin <dev@kresin.me>

package/base-files/files/lib/preinit/10_sysinfo

@@ -3,7 +3,7 @@ do_sysinfo_generic() {
 	mkdir -p /tmp/sysinfo
 	[ -e /tmp/sysinfo/board_name ] || \
 		echo "$(strings /proc/device-tree/compatible | head -1)" > /tmp/sysinfo/board_name
-	[ -n /tmp/sysinfo/model -a -e /proc/device-tree/model ] && \
+	[ ! -e /tmp/sysinfo/model -a -e /proc/device-tree/model ] && \
 		echo "$(cat /proc/device-tree/model)" > /tmp/sysinfo/model
 }