configs: Migrate the various SPL_BOOT_xxx choices for PowerPC

The non-CONFIG_SPL_FRAMEWORK SPL used on some PowerPC platforms have a
choice between CONFIG_SPL_NAND_BOOT, CONFIG_SPL_MMC_BOOT and
CONFIG_SPL_SPI_BOOT.  Migrate this to Kconfig

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/BSC9131RDB_NAND_SYSCLK100_defconfig b/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
index 86073bf..b789f90 100644
--- a/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
+++ b/configs/BSC9131RDB_NAND_SYSCLK100_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_MISC_INIT_R is not set
 # CONFIG_SPL_FRAMEWORK is not set
+CONFIG_SPL_NAND_BOOT=y
 CONFIG_SPL_TEXT_BASE=0xFFFFE000
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y