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/pcm051_rev3_defconfig b/configs/pcm051_rev3_defconfig
index 61102e0..b14754f 100644
--- a/configs/pcm051_rev3_defconfig
+++ b/configs/pcm051_rev3_defconfig
@@ -1,6 +1,5 @@
 CONFIG_ARM=y
 CONFIG_AM33XX=y
-CONFIG_TARGET_PCM051=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_PCM051=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="REV3"
@@ -20,6 +20,7 @@
 CONFIG_SPL_ETH_SUPPORT=y
 CONFIG_SPL_NET_SUPPORT=y
 CONFIG_SPL_NET_VCI_STRING="pcm051 U-Boot SPL"
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set