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/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index 9dba3f0..89c45da 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -1,6 +1,5 @@
 CONFIG_ARM=y
 CONFIG_AM33XX=y
-CONFIG_TARGET_AM335X_SL50=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
@@ -12,6 +11,7 @@
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
+CONFIG_TARGET_AM335X_SL50=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_SPL_YMODEM_SUPPORT=y
@@ -20,6 +20,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_CMD_BOOTZ=y