beaglebone: Use a relative path for systemd's unit file

...so that it's clearly visible in the target FS, and so that it matches
the other symlink which happens to be present in the rootfs, too.

Change-Id: I9001aaabf81af3a25c94322a1767224e00f33961
diff --git a/board/czechlight/beaglebone/overlay/etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service b/board/czechlight/beaglebone/overlay/etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service
index 252308a..51e45a4 120000
--- a/board/czechlight/beaglebone/overlay/etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service
+++ b/board/czechlight/beaglebone/overlay/etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service
@@ -1 +1 @@
-/lib/systemd/system/serial-getty@.service
\ No newline at end of file
+../../../../lib/systemd/system/serial-getty@.service
\ No newline at end of file