Build: Only preopt boot images in eng builds
Build: Only preopt boot images in eng builds

As a middle way between full preopt/high performance/long builds,
and no preopt/low performance/fast turnaround, preopt only the
boot image in eng builds.

Bug: 26794212
Change-Id: I4a2692f3ce84823cd40c6b7d672fd73257739ef8

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build