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/Android.mk b/target/board/Android.mk
index 64e3a74..fc2f651 100644
--- a/target/board/Android.mk
+++ b/target/board/Android.mk
@@ -20,11 +20,8 @@
   INSTALLED_KERNEL_TARGET :=
 endif
 
-ifneq ($(strip $(TARGET_NO_RADIOIMAGE)),true)
-  INSTALLED_RADIOIMAGE_TARGET := $(PRODUCT_OUT)/radio.img
-else
-  INSTALLED_RADIOIMAGE_TARGET :=
-endif
+# Use the add-radio-file function to add values to this variable.
+INSTALLED_RADIOIMAGE_TARGET :=
 
 ifeq (,$(wildcard $(TARGET_DEVICE_DIR)/AndroidBoard.mk))
   ifeq (,$(wildcard $(TARGET_DEVICE_DIR)/Android.mk))