image_decompress: add APIs for decompressing images ...

These APIs are used by platforms that need to decompress images.

image_decompress_init():
  This registers a temporary buffer and a decompressor callback.
  This should be called from platform init code.

image_decompress_prepare():
  This should be called before each compressed image is loaded.  The
  best location to call this will be bl*_plat_handle_pre_image_load().

image_decompress():
  This should be called after each compressed image is loaded.  The
  best location to call this will be bl*_plat_handle_post_image_load().

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