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/ipam390_defconfig b/configs/ipam390_defconfig
index 603b41b..f80b059 100644
--- a/configs/ipam390_defconfig
+++ b/configs/ipam390_defconfig
@@ -10,6 +10,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot > "
 # CONFIG_CMD_IMLS is not set