recovery: install packages in a known mount environment

When installing a package, we should have /tmp and /cache mounted and
nothing else.  Ensure this is true by explicitly mounting them and
unmounting everything else as the first step of every install.

Also fix an error in the progress bar that crops up when you do
multiple package installs in one instance of recovery.

Change-Id: I4837ed707cb419ddd3d9f6188b6355ba1bcfe2b2
diff --git a/roots.h b/roots.h
index 8abe18f..230d9de 100644
--- a/roots.h
+++ b/roots.h
@@ -42,6 +42,10 @@
 // it is mounted.
 int format_volume(const char* volume);
+// Ensure that all and only the volumes that packages expect to find
+// mounted (/tmp and /cache) are mounted.  Returns 0 on success.
+int setup_install_mounts();
 #ifdef __cplusplus