Kconfig: fix BUILD_TARGET for ARCH_MVEBU

Commit dc146ca1

 ("Kconfig: Migrate CONFIG_BUILD_TARGET") made the
mvebu default build target depend on CONFIG_SPL_BUILD. Unfortunately,
there is no such Kconfig symbol. Use the CONFIG_SPL symbol instead to
fix that.

Cc: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Stefan Roese <sr@denx.de>