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

Platform specific BootROM may use its own image type for loading SPL or
U-Boot proper. In some cases it makes sense to not use BootROM supplied
code for booting U-Boot proper but rather to use U-Boot SPL for this,
e.g. when U-Boot SPL can load U-Boot proper faster than BootROM. In this
case it is required for platform board code to parse and load U-Boot in
BootROM specific image type.

This change adds support for parsing platform / board / BootROM specific
image types via weak function spl_parse_board_header() which is called
before marking boot image as a raw.

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