GE Bx50v3 boards: fix fdt file variable

The CONFIG_DEFAULT_FDT_FILE expects just the file not a full path
as that might vary depending on the variant of Linux distro you
might be using.

Cc: Martin Donnelly <martin.donnelly@ge.com>
Cc: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Fabio Estevam <fabio.estevam@nxp.com>