remove the notion of "root path"; support mixed flash types

Remove the wacky notion of "roots" and "root paths" (those things that
look like "FOO:some/path" instead of just "/foo/some/path").  Let each
device specify its own table of available partitions and how to mount
them (needed for devices that use both MTD/yaffs2 and EMMC/ext4

(Cherrypicked from gingerbread w/slight edits.)

Change-Id: I2479ce76b13e73f1d12035c89386c3a82b3edf51
diff --git a/common.h b/common.h
index 0c250f4..97e87ee 100644
--- a/common.h
+++ b/common.h
@@ -87,4 +87,17 @@
 #define STRINGIFY(x) #x
 #define EXPAND(x) STRINGIFY(x)
+typedef struct {
+    const char* mount_point;  // eg. "/cache".  must live in the root directory.
+    const char* fs_type;      // "yaffs2" or "ext4" or "vfat"
+    const char* device;       // MTD partition name if fs_type == "yaffs"
+                              // block device if fs_type == "ext4" or "vfat"
+    const char* device2;      // alternative device to try if fs_type
+                              // == "ext4" or "vfat" and mounting
+                              // 'device' fails
+} Volume;