x86: Rename board_final_cleanup() to board_final_init()

This function sounds like something that is called when U-Boot is about to
jump to Linux. In fact it is an init function.

Rename it to reduce confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>