Nokia RX-51: Simplify calculation of attached kernel image address

Now when board starup code does not copy image to CONFIG_SYS_TEXT_BASE
address there is no need to calculate all addresses from base address at
runtime. The only address which needs to be calculated is attached kernel
image address which can be simplified at compile time without need to know
CONFIG_SYS_TEXT_BASE address or relocation address at the runtime.

Signed-off-by: Pali Rohár <pali@kernel.org>