Add support for building boost using ndk provided in Android sources

Add possibility to build boost for android when building android source code. 

Android source code has it's own NDK tree organization different from "official" NDK.

Now when build-android.sh is called without parameters it checks if ANDROID_BUILD_TOP exists (from android sources build system). If this variable exists it set a flag "AndroidSourcesDetected".

When flag AndroidSourcesDetected is set:
 - NDK version is detected based on "${ANDROID_BUILD_TOP}/ndk/docs/CHANGES.html" as RELEASE.TXT does not exists
 - CXXPATH is overwriten to "${ANDROID_TOOLCHAIN}/arm-linux-androideabi-g++". (ANDROID_TOOLCHAIN comes from android sources build system)