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/cairo_defconfig b/configs/cairo_defconfig
index e6c0b80..ba13212 100644
--- a/configs/cairo_defconfig
+++ b/configs/cairo_defconfig
@@ -5,6 +5,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Cairo # "
 CONFIG_CMD_BOOTZ=y