Build system: Trigger dependency checking only for build targets

The Makefile used to specify a blacklist of rules for which
dependency checking must not be triggered.  This list included
cleaning rules only, whereas all other non-build targets (e.g.
help, checkpatch, etc.) should also be included.

This approach seems a bit fragile because it is easy to forget
some non-building rules in the blacklist, as the experience
showed us.  It is more robust to specify a whitelist of rules
for which dependency checking is required.

Fixes ARM-software/tf-issues#112

Change-Id: I030c405abb35972a726a5200396430316d18f963