nyan: Add a "special-class" for aggregating BCT files into bct.cfg.

The config file which cbootimage processes to create a BCT could come from
multiple different files, individually selected based on config options,
and/or split up into different files for organizational purposes. This change
adds a special-class which collects those files and concatenates them all
together in a bct.cfg which can be processed more easily by other parts of the
build.

While the BCT files themselves are potentially very board specific, for
instance ones that hold memory timing information, this bit of code which
collects them is not. It has to be in each board file instead of alongside the
CPU, however, to ensure that the special class is set up before another
Makefile tries to use it. If we end up with lots of Tegra based boards which
duplicate this code over and over, we might want to revisit how this works.

BUG=None
TEST=Built for nyan while forcing the bct.cfg rule to be executed. Verified
that the bct.cfg was created successfully. With other changes, used the
bct.cfg in an image.
BRANCH=None

Change-Id: I58e1373434f89e69298990ea4643a19d8afdc309
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/170922
Reviewed-by: Gabe Black <gabeblack@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>

src/mainboard/google/nyan/Makefile.inc

@@ -17,6 +17,16 @@
 ## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 ##
 
+# Add a handler for BCT config files
+$(call add-special-class,bct-cfg)
+bct-cfg-handler= $(eval $(obj)/generated/bct.cfg: $(1)$(2))
+
+$(obj)/generated/bct.cfg:
+	@printf "    CAT        $(subst $(obj)/,,$(@))\n"
+	cat $^ > $@
+
+subdirs-y += bct
+
 romstage-y += romstage.c
 
 ramstage-y += mainboard.c