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/core/definitions.mk b/core/definitions.mk
index 1d9dd74..69c7aae 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1569,6 +1569,25 @@
  )
 endef
 
+
+###########################################################
+## Define device-specific radio files
+###########################################################
+
+# Copy a radio image file to the output location, and add it to
+# INSTALLED_RADIOIMAGE_TARGET.
+# $(1): filename
+define add-radio-file
+  $(eval $(call add-radio-file-internal,$(1)))
+endef
+define add-radio-file-internal
+INSTALLED_RADIOIMAGE_TARGET += $$(PRODUCT_OUT)/$(1)
+ALL_PREBUILT += $$(PRODUCT_OUT)/$(1)
+$$(PRODUCT_OUT)/$(1) : $$(LOCAL_PATH)/$(1) | $$(ACP)
+	$$(transform-prebuilt-to-target)
+endef
+
+
 ###########################################################
 ## Other includes
 ###########################################################