lib/sysroot: Maintain one variable for "root is ostree booted"

This was pointed out in a previous PR review; we don't have
a need for the separate variables.  Prep for adding an API for
this.

Closes: #1568
Approved by: jlebon

src/libostree/ostree-sysroot-private.h

@@ -50,8 +50,7 @@ struct OstreeSysroot {
   GLnxLockFile lock;
 
   gboolean loaded;
-  gboolean ostree_booted;
-  gboolean root_is_sysroot; /* TRUE if sysroot_fd is pointed to rootfs "/" */
+  gboolean root_is_ostree_booted; /* TRUE if sysroot is / and we are booted via ostree */
   /* The device/inode for /, used to detect booted deployment */
   dev_t root_device;
   ino_t root_inode;