sdk: add DLL_EXTENSION as an atree variable

This patch defines the DLL_EXTENSION variable to the host
shared library suffix (i.e. either ".so", ".dll" or ".dylib")
when packaging the SDK.

This is required by another patch to sdk/build/tools.atree that
will use ${DLL_EXTENSION} to copy the GLES emulation host libraries
to $SDK/tools/lib

Change-Id: I120d39e905afb453b0702d5f0eb2707f214385cc