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_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index c6ef650..60d2851 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -1,7 +1,7 @@
 CONFIG_ARM=y
 CONFIG_AM33XX=y
-CONFIG_TARGET_AM335X_EVM=y
 # CONFIG_SPL_NAND_SUPPORT is not set
+CONFIG_TARGET_AM335X_EVM=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DEFAULT_DEVICE_TREE="am335x-boneblack"
 CONFIG_FIT=y
@@ -12,6 +12,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"