One typical usage of the pre-tool image filter is data compression,
and GZIP is one of the most commonly used compression methods.
I guess this is generic enough to be put in the common script instead
of platform.mk.

If you want to use this, you can add something like follows to your
platform.mk:

    BL32_PRE_TOOL_FILTER := GZIP
    BL33_PRE_TOOL_FILTER := GZIP

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>