arm: s5p4418: fix relocation of vectors

The header (NSIH) used for the s5p4418-SoC is not loaded into RAM by the
2nd-bootloader, see boot0.h. Therefore, use an adapted version of
relocate_vectors which relocates the vectors after the header (at _start)
instead of the 'dummy'-vectors at the start of the header (at
__image_copy_start).

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>