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/a3m071_defconfig b/configs/a3m071_defconfig
index 9e573a7..5356489 100644
--- a/configs/a3m071_defconfig
+++ b/configs/a3m071_defconfig
@@ -10,6 +10,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
 CONFIG_SPL_NOR_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_LOOPW=y
 # CONFIG_CMD_SETEXPR is not set