Remove device tree booting dependency on CONFIG_SYS_BOOTMAPSZ

The previous patch makes u-boot use the full accessible size of ram as
the default boot mapped size if CONFIG_SYS_BOOTMAPSZ is not defined,
which means boot_relocate_fdt() can be changed to depend solely on
CONFIG_OF_LIBFDT.

Signed-off-by: Grant Likely <grant.likely@linaro.org>