arm: mach-omap2: Move common image process functions out of board files

The functions board_fit_image_post_process() and board_tee_image_process()
are not actually board specific (despite their names). Any board using the
OMAP2 family can use these functions. Move them to boot-common.c.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>