jenkins: Make the multistrap release retargetable

This will allow us to build rootfs images from the apt repo release that we
want, rather than requiring us to build the whole filesystem each time.

Change-Id: Ida333b97b83b2571666e256b44a3715f7572cf35
diff --git a/docker.mk b/docker.mk
index a0bc50c..8244c96 100644
--- a/docker.mk
+++ b/docker.mk
@@ -44,6 +44,7 @@
   -e "HEADLESS_BUILD=$(HEADLESS_BUILD)" \
   -e "IS_JENKINS=$(IS_JENKINS)" \
   -e "IS_EXTERNAL=$(IS_EXTERNAL)" \
+  -e "RELEASE_NAME=$(RELEASE_NAME)" \
   -e "http_proxy=$(http_proxy)" \
   -e "USERSPACE_ARCH=$(USERSPACE_ARCH)" \
   -e "QEMU_ARCH=$(QEMU_ARCH)"
diff --git a/multistrap.mk b/multistrap.mk
index fd35fbd..05daff6 100644
--- a/multistrap.mk
+++ b/multistrap.mk
@@ -43,6 +43,7 @@
 	cp $(ROOTDIR)/board/multistrap.conf $(PRODUCT_OUT)/multistrap
 else
 	cp $(ROOTDIR)/board/multistrap-jenkins.conf $(PRODUCT_OUT)/multistrap/multistrap.conf
+	sed -i -e 's/RELEASE_NAME/$(RELEASE_NAME)/g' $(PRODUCT_OUT)/multistrap/multistrap.conf
 endif
 
 	sed -i -e 's/USERSPACE_ARCH/$(USERSPACE_ARCH)/g' $(PRODUCT_OUT)/multistrap/multistrap.conf
diff --git a/preamble.mk b/preamble.mk
index 763264f..e14b48d 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -20,6 +20,7 @@
 ifneq ($(IS_JENKINS),)
   IS_EXTERNAL = true
   FETCH_PACKAGES = true
+  RELEASE_NAME ?= unstable
 endif
 
 ifeq (,$(wildcard /etc/dpkg/origins/glinux))
diff --git a/rootfs.mk b/rootfs.mk
index 342532a..89cfa77 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -81,6 +81,7 @@
 	cp $(ROOTDIR)/board/multistrap.conf $(PRODUCT_OUT)
 else
 	cp $(ROOTDIR)/board/multistrap-jenkins.conf $(PRODUCT_OUT)/multistrap.conf
+	sed -i -e 's/RELEASE_NAME/$(RELEASE_NAME)/g' $(PRODUCT_OUT)/multistrap.conf
 endif
 
 	sed -i -e 's/MAIN_PACKAGES/$(PACKAGES_EXTRA)/g' $(PRODUCT_OUT)/multistrap.conf