board_f: fix calculation of reloc_off

relocate_code() calculates the relocation offset wrt. the symbol
__image_copy_start which happens to have the same value as
CONFIG_TEXT_BASE on most systems.
When creating an i.MX boot image with an integrated IVT it is
convenient to have CONFIG_TEXT_BASE point to the start of the IVT
that is prepended to the actual code. Thus CONFIG_TEXT_BASE will
differ from __image_copy_start, while the calculation
'gd->relocaddr - __image_copy_start' still gives the right relocation
offset.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>