bootm: Reduce the unnecessary memmove

Although load address and image start address are same address,
bootm command always does memmove.
That is unnecessary memmove and can be taken few milliseconds
(about 500 msec to 1000 msec).
If skip this memmove, we can reduce the boot time.

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>