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This patch adds a new script named 'gen-platforms.sh', used
like 'build-platforms.sh' to generate final platform sysroot
directories. They take the same parameters.

The difference between these two scripts is that gen-platforms.sh
does generate shared libraries from functions/variables list files
found in development/ndk/platforms/android-$PLATFORM/arch-$ARCH/symbols/

built-platforms.sh instead copies the files from .../lib directory
instead.

In a future patch, we're going to remove all prebuilt shared libraries
from development/ndk, and replace them with symbol files. We will also
remove build-platforms.sh and replace its usage by gen-platforms.sh

Change-Id: I2720095f0fcbbbfb5e1374cd1fdaac898ec53393