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/twister_defconfig b/configs/twister_defconfig
index f419140..c92bfc8 100644
--- a/configs/twister_defconfig
+++ b/configs/twister_defconfig
@@ -5,6 +5,7 @@
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="twister => "
 # CONFIG_CMD_IMLS is not set