Adds Cloud Apt as backup sources

* In case mendel-linux.org is down, use cloud.google.com for backup.

Change-Id: Ifc710bc266f84e6e984bb786284ace058a76438d
diff --git a/multistrap.conf b/multistrap.conf
index 43fc4bd..f3c3953 100644
--- a/multistrap.conf
+++ b/multistrap.conf
@@ -2,8 +2,8 @@
 arch=USERSPACE_ARCH
 cleanup=true
 unpack=true
-bootstrap=main bsp edgetpu
-aptsources=main bsp edgetpu
+bootstrap=main bsp edgetpu main_backup bsp_backup
+aptsources=main bsp edgetpu main_backup bsp_backup
 noauth=true
 
 [main]
@@ -20,3 +20,13 @@
 packages=
 source=https://packages.cloud.google.com/apt
 suite=coral-edgetpu-stable
+
+[main_backup]
+packages=MAIN_PACKAGES
+source=https://packages.cloud.google.com/apt/mendel-RELEASE_NAME
+suite=RELEASE_NAME
+
+[bsp_backup]
+packages=
+source=https://packages.cloud.google.com/apt/mendel-RELEASE_NAME-bsp-enterprise
+suite=RELEASE_NAME