pkg-infra: warn about use of deprecated GENTARGETS

With the replacement of GENTARGETS by package-generic, there is a risk
that local packages don't work anymore without any indication of what
is wrong.  Therefore, generate an error message if the GENTARGETS,
AUTOTARGETS or CMAKETARGETS macro is still used.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>