The builtin .dtb.S intermediate file needs to be marked with .SECONDARY
so that it isn't automatically deleted (which causes it to be
regenerated on every build). Also add *.dtb.S to clean-files so it gets
cleaned up by make clean.

Similarly, if the specified builtin dtb isn't already in dtb-y (e.g.
imported into the tree and specified in CONFIG_METAG_BUILTIN_DTB_NAME)
it too will be treated as an intermediate and deleted automatically
(again causing it to be regenerated on every build), so add it to dtb-y
so it gets added to targets and the dtbs target.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>