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>