kwbimage: Rename CONFIG_SYS_SPI_U_BOOT_OFFS to CONFIG_SYS_U_BOOT_OFFS

To use this offset for other boot device (like SDIO/MMC), lets rename
it to a more generic name. This will be used be the SDIO/MMC SPL boot
support for the A38x.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <eibach@gdsys.de>