Don't clean INSTALLED_RADIOIMAGE_TARGET in target/board/Android.mk

We are deprecating use of AndroidBoard.mk. To move add-radio-file to
Android.mk, cleaning INSTALLED_RADIOIMAGE_TARGET may clean the radio
target set by other Android.mks that get included before
target/board/Android.mk.
Bug: 3500660

Change-Id: Iee19c40c18f0e9ec23bad4b6ab0e600756af615a
1 file changed