spl: move SPL_OS_BOOT to Kconfig

Move SPL_OS_BOOT to Kconfig and cleanup existing
uses.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/configs/am57xx_evm_nodt_defconfig b/configs/am57xx_evm_nodt_defconfig
index 608cc10..9588a55 100644
--- a/configs/am57xx_evm_nodt_defconfig
+++ b/configs/am57xx_evm_nodt_defconfig
@@ -7,6 +7,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_DMA_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set