Rotate logs only when there are actual operations

Currently it rotates the log files every time it boots into the recovery
mode. We lose useful logs after ten times. This CL changes the rotation
condition so that it will rotate only if it performs some actual
operations that modify the flash (installs, wipes, sideloads and etc).

Bug: 19695622
Change-Id: Ie708ad955ef31aa500b6590c65faa72391705940
diff --git a/common.h b/common.h
index 4f1c099..b818ceb 100644
--- a/common.h
+++ b/common.h
@@ -40,6 +40,7 @@
 #define STRINGIFY(x) #x
 #define EXPAND(x) STRINGIFY(x)
+extern bool modified_flash;
 typedef struct fstab_rec Volume;
 // fopen a file, mounting volumes and making parent dirs as necessary.