sysroot: Add a public #define OSTREE_PATH_BOOTED

This is public API.  Motivated by
https://github.com/coreos/rpm-ostree/pull/3325/files#diff-56528694f6f3213d6fb88d872f77291412dceec263b57166519843b13eca9a4dR30

src/libostree/ostree-sysroot.c

@@ -973,7 +973,7 @@ ostree_sysroot_initialize (OstreeSysroot  *self,
        * we'll use it to sanity check that we found a booted deployment for example.
        * Second, we also find out whether sysroot == /.
        */
-      if (!glnx_fstatat_allow_noent (AT_FDCWD, "/run/ostree-booted", NULL, 0, error))
+      if (!glnx_fstatat_allow_noent (AT_FDCWD, OSTREE_PATH_BOOTED, NULL, 0, error))
         return FALSE;
       const gboolean ostree_booted = (errno == 0);
 
@@ -1106,11 +1106,11 @@ sysroot_load_from_bootloader_configs (OstreeSysroot  *self,
         return FALSE;
       if (errno == ENOENT)
         {
-          return glnx_throw (error, "Unexpected state: /run/ostree-booted found, but no /boot/loader directory");
+          return glnx_throw (error, "Unexpected state: %s found, but no /boot/loader directory", OSTREE_PATH_BOOTED);
         }
       else
         {
-          return glnx_throw (error, "Unexpected state: /run/ostree-booted found and in / sysroot, but bootloader entry not found");
+          return glnx_throw (error, "Unexpected state: %s found and in / sysroot, but bootloader entry not found", OSTREE_PATH_BOOTED);
         }
      }