images: move board variable to parent makefile ...

A lot of the image makefiles define an equal board variable, which
gives the impression that this variable is unique for this makefile.
As those files aren't freestanding makefiles but get included into a
parent makefile this is not actually true. Attempts to override this
variable will not work reliable as make is picking up a random instance.

Fix this confusion by moving this variable out of the individual makefiles.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

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>