Fix rootfs multistrap

- Okay, seems like there's enough subtle differences between a modern
system and the old Ubuntu on Kokoro, even though we're in a container.
Specifically, it seems like binfmt_support behaves a bit differently. To
fix the errors trying to run dash's preinst script (and presumably,
anything farther down as well), deploy the qemu binary into the chrooted
filesystem, and after the configuring of the system is finished, remove
it.

Change-Id: I79b2c12b035c28ad1a0bd9b06bb3ef430f3ec60d
diff --git a/kokoro/rootfs.sh b/kokoro/rootfs.sh
index 61d65e0..fe5f5c3 100644
--- a/kokoro/rootfs.sh
+++ b/kokoro/rootfs.sh
@@ -12,7 +12,6 @@
 export FETCH_PACKAGES=false
 export PREBUILT_DOCKER_ROOT=$KOKORO_GFILE_DIR
 
-m prereqs
 m docker-rootfs_raw
 
 cp git/out/target/product/imx8m_phanbell/obj/ROOTFS/rootfs.raw.img $KOKORO_ARTIFACTS_DIR
diff --git a/rootfs.mk b/rootfs.mk
index 98ed474..72263a5 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -57,7 +57,7 @@
 	mkdir -p $(dir $(ROOTFS_RAW_IMG))
 	cp $< $<.sha256sum $(dir $(ROOTFS_RAW_IMG))
 else
-$(ROOTFS_RAW_IMG): $(ROOTDIR)/build/preamble.mk $(ROOTDIR)/build/rootfs.mk
+$(ROOTFS_RAW_IMG): $(ROOTDIR)/build/preamble.mk $(ROOTDIR)/build/rootfs.mk /usr/bin/qemu-aarch64-static
 	mkdir -p $(ROOTFS_DIR)
 	rm -f $(ROOTFS_RAW_IMG)
 	fallocate -l 4G $(ROOTFS_RAW_IMG)
@@ -71,8 +71,10 @@
 	sudo multistrap -f $(PRODUCT_OUT)/multistrap.conf -d $(ROOTFS_DIR)
 
 	sudo mount -o bind /dev $(ROOTFS_DIR)/dev
+	sudo cp /usr/bin/qemu-aarch64-static $(ROOTFS_DIR)/usr/bin
 	sudo chroot $(ROOTFS_DIR) /var/lib/dpkg/info/dash.preinst install
 	sudo DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true LC_ALL=C LANGUAGE=C LANG=C chroot $(ROOTFS_DIR) dpkg --configure -a
+	sudo rm -f $(ROOTFS_DIR)/usr/bin/qemu-aarch64-static
 	sudo umount $(ROOTFS_DIR)/dev
 	sudo umount $(ROOTFS_DIR)
 	sudo rmdir $(ROOTFS_DIR)