Changes

Makefile: avoid fetching deps if not needed

Running `make deps` for the first time fetches all the dependencies.
Running `make deps` again currently results in unnecessary git-fetch
and get-checkout operations for each dependency.

This is not as bad as cloning dependencies from scratch every time,
but still takes time and triggers unnecessary requests to the servers.
It's also creating problems when building testsuites offline.

This patch makes the build system a bit smarter in a way that it would
only try to update dependencies if the 'deps/Makefile' was changed.

Change-Id: Iae98585f0a83b17596857eba8c2f51a785198580
Related: OS#6572