board_f: Only reserve memory for U-Boot if we're going to relocate

board_f: Only reserve memory for U-Boot if we're going to relocate

In case of no relocation we'll just waste some space at the very end
of usable memory area. If target device has very limited amount of memory
(for example 256 kB) this loss will be pretty inconvenient.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefan Roese <sr@denx.de>