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/board/ti/ks2_evm/board_k2hk.c b/board/ti/ks2_evm/board_k2hk.c
index c733099..e99e635 100644
--- a/board/ti/ks2_evm/board_k2hk.c
+++ b/board/ti/ks2_evm/board_k2hk.c
@@ -150,7 +150,7 @@
 }
 #endif
 
-#if defined(CONFIG_FIT_EMBED)
+#if defined(CONFIG_MULTI_DTB_FIT)
 int board_fit_config_name_match(const char *name)
 {
 	if (!strcmp(name, "keystone-k2hk-evm"))