Automatic download of NDK/SDK

Add targets download-sdk and download-ndk to fetch required files from the web.
By default, files will be saved in the current directory. A check ensures the
presence of required directories after download and extraction of archives.
Defining SDK and NDK in fill_out.mk overrides the default location. There
exists a make clean target to remove default SDK and NDK locations.

BUILD_PLATFORM is now also guessed automatically, but can be overriden in
fill_out.mk.