tf300t: allow overriding partition for boot image

 Depends-On: android_build~android-5.0~Ie97c8f6821c30aa4b3e7c5249fbd98bd9b798ffd
 https://gerrit.omnirom.org/#/c/10910/

Change-Id: If81535b4825715443004fe5b6aa226fc5db9fbfd