moveconfig: prepare moving CONFIG_SYS_SPI_U_BOOT_OFFS to Kconfig step 2

some boards have common headers for several individual build-targets
where CONFIG_SYS_SPI_U_BOOT_OFFS is defined even it is not needed (only
needed if CONFIG_SPL_SPI_LOAD is defined also). Take this define here
under '#ifdef CONFIG_SPL_SPI_LOAD' for having a clean run of
moveconfig.py

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>