sh: add shared relocate_code() function and call board_init_r()

Commits b61e90e6 ("sh: Drop the arch-specific board init") and
f41e6088 ("sh: Fix build errors for generic board") left code and
data relocation done in start.S, however further actual U-boot
configuration is not started anymore. Practically SH boards with the
code relocated into the expected position by start.S still can be
booted, so the change adds this option and provides an option how to
relocate code for board_init_r() execution.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>