Convert CONFIG_SKIP_LOWLEVEL_INIT et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SKIP_LOWLEVEL_INIT
   CONFIG_SKIP_LOWLEVEL_INIT_ONLY

In order to do this, we need to introduce SPL and TPL variants of these
options so that we can clearly disable these options only in SPL in some
cases, and both instances in other cases.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/tcl/sl50/board.c b/board/tcl/sl50/board.c
index 24bbcd4..b7ddc3b 100644
--- a/board/tcl/sl50/board.c
+++ b/board/tcl/sl50/board.c
@@ -40,7 +40,7 @@
 
 static struct ctrl_dev *cdev = (struct ctrl_dev *)CTRL_DEVICE_BASE;
 
-#ifndef CONFIG_SKIP_LOWLEVEL_INIT
+#if !CONFIG_IS_ENABLED(SKIP_LOWLEVEL_INIT)
 
 static const struct ddr_data ddr3_sl50_data = {
 	.datardsratio0 = MT41K256M16HA125E_RD_DQS,