WIRESHARK_TARGET_PLATFORM is only used inside --download code path:
  Use it only there.
Update comments wrt DOWNLOAD_PREFIX
Remove trailing / at end of DOWNLOAD_PREFIX

Change-Id: Ia161d4226ff3d463bceaaf4e01de70b2db98bf3c
Reviewed-on: https://code.wireshark.org/review/3142
Reviewed-by: Jörg Mayer <jmayer@loplof.de>