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/xilinx_zynqmp_zc1751_xm018_dc4_defconfig b/configs/xilinx_zynqmp_zc1751_xm018_dc4_defconfig
index ec55a6d..2feee77 100644
--- a/configs/xilinx_zynqmp_zc1751_xm018_dc4_defconfig
+++ b/configs/xilinx_zynqmp_zc1751_xm018_dc4_defconfig
@@ -11,6 +11,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
+CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="ZynqMP> "
 # CONFIG_CMD_IMLS is not set