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/devkit8000_defconfig b/configs/devkit8000_defconfig
index e836e5e..39a35e9 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_DEVKIT8000=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set