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/Makefile b/Makefile
index 1a2ef30..888486b 100644
--- a/Makefile
+++ b/Makefile
@@ -878,7 +878,7 @@
 quiet_cmd_copy = COPY    $@
       cmd_copy = cp $< $@
 
-ifeq ($(CONFIG_FIT_EMBED),y)
+ifeq ($(CONFIG_MULTI_DTB_FIT),y)
 
 fit-dtb.blob: dts/dt.dtb FORCE
 	$(call if_changed,mkimage)