build: Fix OTA package generation with multi-image uboot files

The ota script generator replicates the mkbootimg invocation (and so
needed to get a mkimage invocation of its own)
Without this, OTA packages were getting standard android bootimages
(which flashed correctly, but of course failed to boot)

Change-Id: If94a20c9d0971a412832f2b77f11535a9b664aa8