unpackbootimg: add a target specifically for unpackbootimg.

for some reason, and I am guessing this is a bug in the Android build, I can no longer run the following:
make -j32 out/host/darwin-x86/bin/unpackbootimg

It works if the build has been built, but not if it is clean. Somewhere within the bowels of AOSP
something broken in 4.1.2.

Change-Id: I4f8c434f3fce9ddaee4f54febba02efac07a31a3