Only call make_recovery_patch when BUILDING_SYSTEM_IMAGE.

The recovery patch gets put in the SYSTEM directory. Placing the
recovery patch here doesn't make sense when not building the system
image, as is the case for merged (system + vendor) builds.

Bug: 128838154
Test: Running make droid dist for a device target that sets PRODUCT_BUILD_SYSTEM_IMAGE to false.
Change-Id: Ib5ce8c8490024199f82d0c093e9a7ae2de5f71f5