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/gwventana_defconfig b/configs/gwventana_defconfig
index acf9b84..ee823d2 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_DMA_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Ventana > "
 CONFIG_CMD_BOOTZ=y