Store the path to the ota file generator where it can be reused

Change-Id: I8896713c79a751b79fbbcc75f6ba30dad4ea9fac

Conflicts:

	core/Makefile
Store the value from TARGET_OTA_ASSERT_DEVICE to a file

This is needed so it can be read in when re-signing a package to
allow for the proper device asserts.

Change-Id: I8f3bb491bbbaa4d2b827a45ef83f12c6c83e6712
Store the value from BOARD_CUSTOM_BOOTIMG_MK to a file

This is needed so it can be read in when re-signing a package to
use the proper boot image.

Print notice when using a custom bootimage makefile

Change-Id: I6baab9f8964ce5d4fa9487f2859af9c385d7b21b
releasetools: don't create prebuilt_dir path if it exits

This path may already exists from previous build tasks. Attempting
to create it again will cause an error.

Change-Id: I4baad8bc846470b99f6ce9207c0682401406decc
Fix droiddoc generation.

We want javadoc generated from the standard libraries
we supply and not the host standard libraries.

This also has the side effect of fixing javadoc generation
for java7 APIs that android introduced, while compiling
with java6.

bug: 8992787
Change-Id: Idebc7e12c7743a43b425ef4971f4482719fd480d