efi: Correct logic for OF_EMBED and OF_SEPARATE co-existing

While we do plan to switch to OF_SEPARATE now it is supported, it seems
worth at least showing how OF_EMBED could be used instead, just for the
record.

So make the Makefile rule conditional on OF_SEPARATE and adjust fdtdec
to avoid a build error when OF_EMBED is used.

Finally. the dtb symbol has a double underscore, so update it to avoid a
build warning.

With future patches, OF_EMBED will no-longer be used with the EFI app,
so it is expected that it will eventually stop working.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 2e7bf25f6b ("Support separate DTB files with the UEFI app")

include/asm-generic/sections.h

@@ -70,7 +70,7 @@ extern char __image_copy_start[], __image_copy_end[];
 extern char __bss_end[];
 extern char __rel_dyn_start[], __rel_dyn_end[];
 extern char _image_binary_end[];
-extern char _dtb[];
+extern char __dtb[];
 
 /*
  * This is the U-Boot entry point - prior to relocation it should be same