dts: renamed FIT_EMBED to MULTI_DTB_FIT and moved it to the dts Kconfig

CONFIG_FIT_EMBED might be confused with CONFIG_OF_EMBED, rename it
MULTI_DTB_FIT as it is able to get a DTB from a FIT image containing
multiple DTBs. Also move the option to the Kconfig dedicated to the DTS
options and create a README for this feature.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 107a892..630e040 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -1217,7 +1217,7 @@
 	else
 		gd->fdt_blob = (ulong *)&__bss_end;
 
-#  elif defined CONFIG_FIT_EMBED
+#  elif defined CONFIG_MULTI_DTB_FIT
 	gd->fdt_blob = locate_dtb_in_fit(&_end);
 
 	if (gd->fdt_blob == NULL || gd->fdt_blob <= ((void *)&_end)) {