generalize the definition of a "radio image"

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.
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index 6ec2de3..2b72d01 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -1,12 +1,11 @@
 # config.mk
-# 
+#
 # Product-specific compile-time definitions.
 #
 
 # The generic product target doesn't have any hardware-specific pieces.
 TARGET_NO_BOOTLOADER := true
 TARGET_NO_KERNEL := true
-TARGET_NO_RADIOIMAGE := true
 TARGET_CPU_ABI := armeabi
 HAVE_HTC_AUDIO_DRIVER := true
 BOARD_USES_GENERIC_AUDIO := true