Revert "fdtdec: Use CONFIG_IS_ENABLED in board_fdt_blob_setup()"

On Rockchip platforms we need this area of code in TPL, but there is no
TPL_SEPARATE_BSS symbol.

This reverts commit 0a2aaab0.
Reported-by: NMarkus Reichl <m.reichl@fivetechno.de>
Reported-by: NJesper Schmitz Mouridsen <jesper@schmitz.computer>
Reported-by: NPeter Robinson <pbrobinson@gmail.com>
Tested-by: NPeter Robinson <pbrobinson@gmail.com>
Signed-off-by: NTom Rini <trini@konsulko.com>

lib/fdtdec.c

@@ -1253,7 +1253,7 @@ __weak void *board_fdt_blob_setup(void)
 	void *fdt_blob = NULL;
 #ifdef CONFIG_SPL_BUILD
 	/* FDT is at end of BSS unless it is in a different memory region */
-	if (CONFIG_IS_ENABLED(SEPARATE_BSS))
+	if (IS_ENABLED(CONFIG_SPL_SEPARATE_BSS))
 		fdt_blob = (ulong *)&_image_binary_end;
 	else
 		fdt_blob = (ulong *)&__bss_end;