Specify squashfs-only in lorax_options

Setting this option will reduce both BOOT.iso and DVD image size
This patch is related to the following change proposal:
https://fedoraproject.org/wiki/Changes/OptimizeSquashFSOnDVDByRemovingEXT4FilesystemImageLayer

Bugzilla ticket:
https://bugzilla.redhat.com/show_bug.cgi?id=1889898

Resolves: rhbz#1889898
Signed-off-by: Bohdan Khomutskyi <bkhomuts@redhat.com>

fedora.conf

@@ -105,11 +105,13 @@ buildinstall_skip = [
 lorax_options = [
   ('^.*$', {
      'x86_64': {
-         'nomacboot': False
+         'nomacboot': False,
+         'squashfs_only': True
      },
      '*': {
          'noupgrade': True,
-         'rootfs_size': 3
+         'rootfs_size': 3,
+         'squashfs_only': True
      }
   })
 ]