layerscape: not set u-boot-dtb.bin in UBOOT_CONFIG
The u-boot binary name is u-boot-fit-dtb.bin if CONFIG_MULTI_DTB_FIT is
set. Below build error appears due to no u-boot-dtb.bin generated:
| cp: cannot stat '.../u-boot-dtb.bin': No such file or directory
| WARNING: exit code 1 from a shell command.
Regardless of CONFIG_OF_SEPARATE or CONFIG_MULTI_DTB_FIT being set, the
binary is copied as u-boot.bin which is used in uboot-config.bbclass by
default.
Not set u-boot-dtb.bin in UBOOT_CONFIG, just use the default u-boot.bin.
