Like on ARM platform keep the first code relocation from a U-boot
image storage to RAM at CONFIG_SYS_TEXT_BASE, then pass execution to a
generic board_init_f() with empty GD flags. If CONFIG_SYS_TEXT_BASE is
equal to a calculated by board_init_f() relocation address there will
be no more code and data copy, however it's worth to mention that the
first copy happens even if $pc on _start is the same as
CONFIG_SYS_TEXT_BASE, on practice this works without a problem.

Also note that _sh_start is renamed back to _start to correct
gd->mon_len calculation by setup_mon_len(), the opposite rename was
done in pre-generic board commit 2024b968

 ("sh: Fix build in start.S").

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