build/nexus: expand builtins into ext_flash.tar

Extract the contents of the builtins cpio bundle and append the files
to the tarball loaded into SPI flash. This will serve as an initial
filesystem for the SEC storage manager.

Change-Id: I7aaab6641f7a400bda3cfa52c8286198326e47a2

platforms/nexus/sim.mk

diff --git a/platforms/nexus/sim.mk b/platforms/nexus/sim.mk
index 38a7c74..a472161 100644
--- a/platforms/nexus/sim.mk
+++ b/platforms/nexus/sim.mk
@@ -49,12 +49,9 @@
 	ln -sf $(CANTRIP_ROOTSERVER_RELEASE) $(TMP_RELEASE)/capdl-loader
 	tar -C $(TMP_RELEASE) -cvhf $@ matcha-tock-bundle.bin kernel capdl-loader
 
-# NB: Package the builtins bundle so it can be written to a carve out in
-#     in SMC memory; this is temporary until the SEC supports returning
-#     the builtins from flash.
+## Packages the builtins in the tarball for writing to SPI flash.
 ext_flash_release: $(EXT_FLASH_RELEASE) $(EXT_BUILTINS_RELEASE) | $(TMP_RELEASE)
-	ln -sf $(EXT_BUILTINS_RELEASE) $(TMP_RELEASE)/cantrip-builtins
-	tar -C $(TMP_RELEASE) -rvhf $(EXT_FLASH_RELEASE) cantrip-builtins
+	tar -C $(CANTRIP_OUT_RELEASE)/builtins -rvhf ${EXT_FLASH_RELEASE} $$(${CPIO} -t < ${EXT_BUILTINS_RELEASE})
 
 # Dredge the platform configuration for the physical address where the
 # cpio archive is expected.