fip: move headers shared between TF and fiptool to include/tools_share

Some header files need to be shared between TF and host programs.
For fiptool, two headers are copied to the tools/fiptool directory,
but it looks clumsy.

This commit introduces a new directory, include/tools_share, which
collects headers that should be shared between TF and host programs.

This will clarify the interface exposed to host tools.  We should
add new headers to this directory only when we really need to do so.

For clarification, I inserted a blank line between headers from the
include/ directory (#include <...>) and ones from a local directory
(#include "..." ).

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