fdt relocate: have more attention to use a bootmap or not

Platforms with flat device tree support can use a bootmap to relocate
the fdt_blob. This is not a must. That's why the relocation function
boot_relocate_fdt() should be use only if CONFIG_OF_LIBFDT was defined
together with CONFIG_SYS_BOOTMAPSZ (see common/cmd_bootm.c).

On MicroBlaze platforms there is no need to use a bootmap to relocate
a fdt blob. So we need a more precise focus on the compilation and usage
of boot_relocate_fdt().

In general it is valid to exclude the function boot_relocate_fdt() if
the bootmap size CONFIG_SYS_BOOTMAPSZ is not defined.

Signed-off-by: Stephan Linz <linz@li-pro.net>