u-boot: support custom u-boot.bin name

On some platforms, the standard U-Boot binary is not called u-boot.bin,
but u-boot<something>.bin. This is for example the case on the
Cavium-provided u-boot, where the output file is called
u-boot_<boardname>.bin.

This patch adds a 'custom format' choice in the existing u-boot binary
format selection, where this name can be put. In the Cavium example, this
option could be set to:
BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot-$(BR2_TARGET_UBOOT_BOARDNAME).bin"

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>