Rather than defining it config.mk we can set it in config.h and remove
config.mk from several boards that don't need it.

We mimic what 4xx does and introduce CONFIG_RESET_VECTOR_ADDRESS for
config.h to set.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Wolfgang Denk <wd@denx.de>