mk: allow application to override clean

In some cases application may want to have additional rules
for clean. This can be handled by allowing the double colon
form of rule.

 https://www.gnu.org/software/make/manual/html_node/Double_002dColon.html

Single colon and double colon rules for same target causes
an error.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>