boot/arm-trusted-firmware: add option BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_BL33_IMAGE

boot/arm-trusted-firmware: add option BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_BL33_IMAGE

Currently, when BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_AS_BL33 is
enabled, the arm-trusted-firmware package assumes that the BL33 image
provided by U-Boot is u-boot.bin. However for some platforms, the name
may be different (for example u-boot-dtb.bin). To support such use
cases, this commit adds a string option
BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_BL33_IMAGE to configure the
U-Boot image name, which defaults to u-boot.bin to preserve backward
compatibility.

Signed-off-by: Jerry Huang <jerry.huang@nxp.com>
Reviewed-by: Sergey Matyukevich <geomatsi@mail.com>
[Thomas: rewrite commit log and Config.in help text]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>