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/igep0030_defconfig b/configs/igep0030_defconfig
index bc0ec6a..4bc0def 100644
--- a/configs/igep0030_defconfig
+++ b/configs/igep0030_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_ONENAND_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set