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/zynq_zc770_xm010_defconfig b/configs/zynq_zc770_xm010_defconfig
index bec60cc..1419efa 100644
--- a/configs/zynq_zc770_xm010_defconfig
+++ b/configs/zynq_zc770_xm010_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SYS_NO_FLASH=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Zynq> "
 # CONFIG_CMD_IMLS is not set