board/freescale/common/imx: make imx8-bootloader-prepare more generic
Allow user to override the hardcoded dtb name by using
BR2_ROOTFS_POST_SCRIPT_ARGS.
The only user of this script, configs/freescale_imx8mqevk, is updated
accordingly.
Cc: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>