package: remove useless arguments from GENTARGETS

Thanks to the pkgparentdir and pkgname functions, we can rewrite the
GENTARGETS macro in a way that avoids the need for each package to
repeat its name and the directory in which it is present.

[Peter: pkgdir->pkgparentdir]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>