arm: Add Kconfig entry to select CONFIG_DTB_FILE

arm: Add Kconfig entry to select CONFIG_DTB_FILE

Currently in order to link existing DTB into Xen image
we need to either specify option CONFIG_DTB_FILE on the
command line or manually add it into .config.
Add Kconfig entry: CONFIG_DTB_FILE
to be able to provide the path to DTB we want to embed
into Xen image. If no path provided - the dtb will not
be embedded.

Remove the line: AFLAGS-y += -DCONFIG_DTB_FILE=\"$(CONFIG_DTB_FILE)\"
as it is not needed since Kconfig will define it in a header
with all the other config options.

Move definition of _sdtb into dtb.S to prevent defining it
if there is no reference to it or if someone protects
_sdtb with #ifdef rather than with .ifnes. If the latter,
we will get a compiler error.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>