Convert CONFIG_SYS_FDT_LOAD_ADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FDT_LOAD_ADDR

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/Kconfig b/arch/sandbox/Kconfig
index 7606469..7cdbaef 100644
--- a/arch/sandbox/Kconfig
+++ b/arch/sandbox/Kconfig
@@ -68,4 +68,14 @@
 	default 32 if HOST_32BIT
 	default 64 if HOST_64BIT
 
+config SYS_FDT_LOAD_ADDR
+	hex "Address at which to load devicetree"
+	default 0x100
+	help
+	  With sandbox the devicetree is loaded into the emulated RAM. This sets
+	  the address that is used. There must be enough space at this address
+	  to load the full devicetree without it overwriting anything else.
+
+	  See `doc/arch/sandbox.rst` for more information.
+
 endmenu
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index b1689d3..fd972b8 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -18,12 +18,6 @@
 
 #define CONFIG_SYS_CBSIZE		1024	/* Console I/O Buffer Size */
 
-/* turn on command-line edit/c/auto */
-
-/* SPI - enable all SPI flash types for testing purposes */
-
-#define CONFIG_SYS_FDT_LOAD_ADDR	        0x100
-
 #define CONFIG_PHYSMEM
 
 /* Size of our emulated memory */
diff --git a/scripts/config_whitelist.txt b/scripts/config_whitelist.txt
index e012dbc..a8c3a38 100644
--- a/scripts/config_whitelist.txt
+++ b/scripts/config_whitelist.txt
@@ -1609,7 +1609,6 @@
 CONFIG_SYS_FAULT_ECHO_LINK_DOWN
 CONFIG_SYS_FAULT_MII_ADDR
 CONFIG_SYS_FDT_BASE
-CONFIG_SYS_FDT_LOAD_ADDR
 CONFIG_SYS_FDT_PAD
 CONFIG_SYS_FEC0_IOBASE
 CONFIG_SYS_FEC1_IOBASE