make sysmocom-archiver work for lc15 kernel

We cannot use 'source_tar_name' in cases it is not defined. Not sure if this is
the right fix, but it at least makes the build succeed.

classes/sysmocom-archiver.bbclass

@@ -460,8 +460,8 @@ python do_archive_linux_yocto(){
     s = d.getVar('S', True)
     if 'linux-yocto' in s:
         source_tar_name = archive_sources(d,'')
-    if d.getVar('SOURCE_ARCHIVE_PACKAGE_TYPE', True).upper() not in 'SRPM':
-        move_tarball_deploy(d,[source_tar_name,''])
+        if d.getVar('SOURCE_ARCHIVE_PACKAGE_TYPE', True).upper() not in 'SRPM':
+            move_tarball_deploy(d,[source_tar_name,''])
 }
 do_kernel_checkout[postfuncs] += "do_archive_linux_yocto "