Adding repack_bootimg into otatools.zip

repack_bootimg is useful to add files into a ramdisk, then repack
a boot.img. Note that mkbootimg, unpack_bootimg, lz4, minigzip, etc.,
that repack_bootimg depends are already in otatools.zip.

Bug: 174443192
Test: `m otatools-package`, unzip $OUT/otatools.zip, export $PATH
      with ./otatools/bin and checks unpack_bootimg can run
Change-Id: Id4fbf2076b50522b95ab46dfeb1e2b7115d6606d