releasetools: use variable instead of fixed name for bootable image

When building for a device with a custom bootimage makefile, this
would cause the boot image to be used as the recovery image during
fastboot zip creation.

Change-Id: I075cfe4e9a2a8384d3767de351ec7f751f3e95ab
Merge remote-tracking branch 'upstream/kk44' into kk44