cmd: add FDT setup for bootelf by flag

Added the ability to use FDT for ELF applications, required to run some
OS. To make FDT setup, you need to set the -d fdt_addr_r cmd option for
bootelf command. Enable by selecting CMD_ELF_FDT_SETUP.

Signed-off-by: Maxim Moskalets <Maxim.Moskalets@kaspersky.com>