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There are cases where we want to process images before they are
passed to cert_create / fiptool.

My main motivation is data compression.  By compressing images, we can
save data storage, and possibly speed up loading images.  The image
verification will also get faster because certificates are generated
based on compressed images.

Other image transformation filters (for ex. encryption), and their
combinations would be possible.  So, our build system should support
transformation filters in a generic manner.

The choice of applied filters is up to platforms (so specified in
platform.mk)

To define a new filter, <FILTER_NAME>_RULE and <FILTER_NAME>_SUFFIX
are needed.

For example, the GZIP compression filter can be implemented as follows:

------------------------>8------------------------
define GZIP_RULE
$(1): $(2)
        @echo "  GZIP    $$@"
        $(Q)gzip -n -f -9 $$< --stdout > $$@
endef

GZIP_SUFFIX := .gz
------------------------>8------------------------

The _RULE defines how to create the target $(1) from the source $(2).
The _SUFFIX defines the extension appended to the processed image path.
The suffix is not so important because the file name information is not
propagated to FIP, but adding a sensible suffix will be good to classify
the data file.

Platforms can specify which filter is applied to which BL image, like
this:

------------------------>8------------------------
BL32_PRE_TOOL_FILTER := GZIP
BL33_PRE_TOOL_FILTER := GZIP
------------------------>8------------------------

<IMAGE_NAME>_PRE_TOOL_FILTER specifies per-image filter.  With this,
different images can be transformed differently.  For the case above,
only BL32 and BL33 are GZIP-compressed.  Nothing is done for other
images.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>