ARM: Rockchip: switch to multiimage support ...

- Add images/Makefile.rockchip
- Allow multiple boards to be selected
- protect initcalls with appropriate of_machine_is_compatible
- rename board specific config to SoC config
- Add dtb file to compilation
- turn barebox_arm_reset_vectorto ENTRY_FUNCTION
- pass dtb to barebox_arm_entry

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>