ota_from_target_files: Validate signatures after possible data wipe

If the data is being wiped, we want to validate the signature after
this step. This is also a good check that the data wipe succeeded.

Change-Id: Iaecfddfb2010944de0c97e7bb21d8e581f20deec
Merge branch 'cm-11.0' of github.com:CyanogenMod/android_build into cm-11.0