mk: fix dependencies when modifying config

If .config file is changed and .depdirs was built before,
it must be updated.
It is now done silently (-s) in checkconfig rule.
Now .depdirs is not redone if .config is older.

It has been tested by enabling PMD_PCAP after a default build.
KVARGS dependency should appear in .depdirs.

There is also a typo fix for PHONY rule.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>