With 778bd9320b ("Makefile.imx: change image creation to
build_imx_habv4img for i.MX6") the barebox image file name for the
board became barebox-barebox-embest-imx6q-marsboard.img.

Drop the extra barebox-.

Fixes: 778bd9320b ("Makefile.imx: change image creation to
       build_imx_habv4img for i.MX6")
Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>