build: Add optional target-specific bootclass jar to dexpreopt

Allow target-specific additional boot class jars via
TARGET_ADDITIONAL_BOOTCLASSPATH for dexpreopt.

Change-Id: I67b31628bc8169f8df09e836b79618aa2a22ed9a
Merge "build: Add optional target-specific bootclass jar to dexpreopt" into jb-mr2
Merge branch 'jb-mr2' of https://github.com/AOKP/build into linaro2