pitx-imx8m: set 'fdtfile' variable for boot script

include/configs/pitx-imx8m.h

@@ -152,6 +152,7 @@
 	"fdt_addr_r=0x43000000\0"						\
 	"fdt_high=0xffffffffffffffff\0"						\
 	"boot_fdt=try\0" \
+	"fdtfile=pitx-imx8m_hdmi.dtb\0"						\
 	"set_fdtfile=setenv fdtfile pitx-imx8m_hdmi.dtb\0"			\
 	"initrd_addr=0x43800000\0"						\
 	"initrd_high=0xffffffffffffffff\0" 					\