releasetools: Fix secondary payload generation.

The change in [1] removed duplicating radio images from RADIO/ to
IMAGES/. When getting the target_files zip for generating secondary
payload, we need to additionally copy the files under RADIO/ over.

[1] commit 5277d101

Test: python -m unittest test_ota_from_target_files
Test: Generate an OTA with secondary payload (--include_secondary).
Change-Id: I096f1642a905fb764e63f5df2edc1396aa6befd8