Convert CONFIG_SPL_INIT_MINIMAL et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_INIT_MINIMAL
   CONFIG_SPL_FLUSH_IMAGE
   CONFIG_SPL_SKIP_RELOCATE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig b/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
index f505a7a..a99f46a 100644
--- a/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
+++ b/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
@@ -32,6 +32,7 @@
 # CONFIG_MISC_INIT_R is not set
 # CONFIG_SPL_FRAMEWORK is not set
 CONFIG_SPL_SPI_BOOT=y
+CONFIG_SPL_FLUSH_IMAGE=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_I2C=y
 CONFIG_SPL_MPC8XXX_INIT_DDR=y