metag: avoid unnecessary builtin dtb rebuilds

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>