systemd's cfg mount generator moved from clearfog to common overlay

Change-Id: I5575f95ada1e935863e5cf7c01d0a3f23c007850
diff --git a/board/czechlight/clearfog/overlay/usr/lib/systemd/system-generators/czehlight-cfg-mount-generator b/board/czechlight/common/overlay/usr/lib/systemd/system-generators/czehlight-cfg-mount-generator
similarity index 100%
rename from board/czechlight/clearfog/overlay/usr/lib/systemd/system-generators/czehlight-cfg-mount-generator
rename to board/czechlight/common/overlay/usr/lib/systemd/system-generators/czehlight-cfg-mount-generator