Move ostree-* executables to /usr/lib/ostree

Why not to use libexecdir?

Because this directory does not exist on some distros or paths
between distros varies. There are several reasons why a well
known path is prefered, for example when generating a kernel
command line:

init=${ostree}/usr/lib/ostree-boot/ostree-prepare-root

In addition this saves us some typing in a console when wanting
to access the "ostree" cmd line.

Closes: #449
Approved by: cgwalters

src/libostree/ostree-bootloader-grub2.c

@@ -315,7 +315,7 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader      *bootloader,
         use_system_grub2_mkconfig = FALSE;
     }
   else
-    grub_exec = use_system_grub2_mkconfig ? GRUB2_MKCONFIG_PATH : LIBEXECDIR "/ostree-grub-generator";
+    grub_exec = use_system_grub2_mkconfig ? GRUB2_MKCONFIG_PATH : "/usr/lib/ostree/ostree-grub-generator";
 
   if (use_system_grub2_mkconfig && ostree_sysroot_get_booted_deployment (self->sysroot) == NULL
       && g_file_has_parent (self->sysroot->path, NULL))