Non-HTC devices may have multiple files constituting their "radio
image". Generalize the INSTALLED_RADIOIMAGE_TARGET variable a bit:
initially define it as empty, then let AndroidBoard.mk files add to
it. Provide a convenience function add-radio-image for them to call
to add files. Put all those files into the target_files zip for use
in OTA and fastboot package construction.
Note that for HTC devices, this changes the name of the radio image in
the target_files zip: instead of "RADIO/image" it will be
"RADIO/radio.img". Tools that use the target_files zip will need to
be changed.
