`distclean-pkg` sub-target only removes PKG_FILE2 when file is default.

erlang.mk packaging instructions encourage user to define a custom package file as follows,

"You can use a custom package file, in which case you will probably want to set the PKG_FILE2 variable to its location."

However, `distclean-pkg` sub-target removes whatever file is defined under PKG_FILE2. If it's a user custom package file, we don't want erlang.mk to remove it.