Booting an image currently sets the environment variable "fdtaddr"
by calling into 'cmd/fdt.c'. As a result, linking U-Boot fails if
CMD_FDT is not enabled.

Fix this by adding 'if (CONFIG_IS_ENABLED(CMD_FDT))' to the two
places where 'set_working_fdt_addr()' is called.
Signed-off-by: NSimon Goldschmidt <simon.k.r.goldschmidt@gmail.com>