add function for device-specific wipe data features

Some devices want to do special things when recovery wipes data (eg,
wipe data in their baseband processor as well).  Add a hook in the
device-specific recovery library that gets called when data is wiped.

Also add an amend root for the "mbm" partition.
diff --git a/roots.c b/roots.c
index 6a6cf8a..8f8dace 100644
--- a/roots.c
+++ b/roots.c
@@ -52,6 +52,7 @@
     { "RECOVERY:", g_mtd_device, NULL, "recovery", "/", g_raw },
     { "SDCARD:", "/dev/block/mmcblk0p1", "/dev/block/mmcblk0", NULL, "/sdcard", "vfat" },
     { "SYSTEM:", g_mtd_device, NULL, "system", "/system", "yaffs2" },
+    { "MBM:", g_mtd_device, NULL, "mbm", NULL, g_raw },
     { "TMP:", NULL, NULL, NULL, "/tmp", NULL },
 };
 #define NUM_ROOTS (sizeof(g_roots) / sizeof(g_roots[0]))