barebox: use the symlink to copy the barebox binary to the images directory

Barebox creates a symlink to the binary. This symlink points to
the barebox.bin or to the compressed zbarebox.bin binary.

Signed-off-by: Gregory Hermant <gregory.hermant@calao-systems.com>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>