odroid: limit boot memory to lowmem

In commit 4963f63f ("image: Use gd->ram_base/_size in
env_get_bootm_size()") the size of the available memory for U-Boot to
use when relocating images, and not otherwise constrained, was changed
to include all known memory banks. In the case of this platform however,
all of the memory known to U-Boot is not also part of the Linux kernel
"lowmem" and so we must use CFG_SYS_BOOTMAPSZ to limit where we relocate
images to. We set a conservative limit of 256MB here to mirror the
previous behavior.

Signed-off-by: Joost van Zwieten <joost@diskos.nl>
[trini: Reword the commit messsage]
Signed-off-by: Tom Rini <trini@konsulko.com>