SPL: Add support for checking board / BootROM specific image types

Commit 9baab60b ("SPL: Add support for parsing board / BootROM specific
image types") added support for loading board specific image types.

This commit adds support for a new weak function spl_parse_board_header()
which is called after loading boot image. Board may implement this function
for checking if loaded board specific image is valid.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>