Change pbuilder to a cross-arm64 instead of native

- In pbuilder, set architecture to amd64 and host-arch to arm64. This
provides an amd64 chroot that cross-compiles for arm64, and as a result
is *much* faster.
- Also, bind mount /var/cache/pbuilder. This way separate invocations of
pbuilder inside docker will still be able to reuse fetched packages.

Change-Id: I686af7e3a2c9f89569b9b2103142199625efc8f2
diff --git a/docker.mk b/docker.mk
index cad77e9..a7a1b19 100644
--- a/docker.mk
+++ b/docker.mk
@@ -46,6 +46,7 @@
 		-v $(PREBUILT_DOCKER_ROOT)\:/docker \
 		-v $(PREBUILT_MODULES_ROOT)\:/modules \
 		-v $(FETCH_PBUILDER_DIRECTORY)\:/pbuilder \
+		-v /var/cache/pbuilder\:/var/cache/pbuilder \
 		-w /rootdir \
 		-e "DEBOOTSTRAP_FETCH_TARBALL=$(DEBOOTSTRAP_FETCH_TARBALL)" \
 		-e "ROOTFS_FETCH_TARBALL=$(ROOTFS_FETCH_TARBALL)" \
diff --git a/packages.mk b/packages.mk
index 4e0f403..df5d6db 100644
--- a/packages.mk
+++ b/packages.mk
@@ -27,8 +27,8 @@
 		--basetgz $@ \
 		--mirror http://http.us.debian.org/debian \
 		--distribution stretch \
-		--architecture arm64 \
-		--extrapackages debhelper
+		--architecture amd64 \
+		--extrapackages crossbuild-essential-arm64 debhelper
 	mkdir -p $(ROOTDIR)/cache/base-tmp
 	cd $(ROOTDIR)/cache/base-tmp; \
 	sudo tar xf $@; \
@@ -74,8 +74,9 @@
 		--buildresult $(PRODUCT_OUT) -- \
 		--basetgz $(ROOTDIR)/cache/base.tgz \
 		--configfile $(ROOTDIR)/build/pbuilderrc \
-		--hookdir $(ROOTDIR)/build/pbuilder-hooks
-	sudo touch $(PRODUCT_OUT)/.$1-pbuilder
+		--hookdir $(ROOTDIR)/build/pbuilder-hooks \
+		--host-arch arm64
+	touch $(PRODUCT_OUT)/.$1-pbuilder
 .PHONY:: $1
 endef