Fix problem of dependencies on the fiptool makefile target

The 'fiptool' target doesn't depend on fip_create's source files,
neither directly nor indirectly. As a result, the FIP tool is not
rebuilt whenever its source files change.

This patch makes the ${FIPTOOL} target into a phony target so that the
FIP tool's sub-makefile is always called. The sub-makefile correctly
handles the dependencies. It also moves the completion message into
the sub-makefile so that it is only displayed when the tool is
actually recompiled.

Fixes ARM-software/tf-issues#278

Change-Id: Ia027519fe51d3c42be30665d1ad20a7b89fa350f
Merge pull request #223 from sandrine-bailleux/sb/fix-fiptool-target

Fix problem of dependencies on the fiptool makefile target