pkg-utils: update comments after renaming of package infra macros

The package infrastructure macros have a long time ago been renamed
from GENTARGETS, AUTOTARGETS to generic-package and
autotools-package. However, one comment in pkg-utils.mk keeps a
reference to these *TARGETS macros. This commit updates this comment
to match the new name of these macros.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>