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/P1021RDB-PC_SPIFLASH_defconfig b/configs/P1021RDB-PC_SPIFLASH_defconfig
index bd91531..19d2b0f 100644
--- a/configs/P1021RDB-PC_SPIFLASH_defconfig
+++ b/configs/P1021RDB-PC_SPIFLASH_defconfig
@@ -19,6 +19,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_BOARD_EARLY_INIT_R=y
 # CONFIG_SPL_FRAMEWORK is not set
+CONFIG_SPL_SPI_BOOT=y
 CONFIG_SPL_TEXT_BASE=0xf8f81000
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y