nanobsd: Update fill_pkg.sh

fill_pkg.sh is a script that links a package and its dependencies from a
"package dump" directory (like /usr/ports/packages/All) to a specified
directory (NANO_PACKAGE_DIR), for cust_pkgng()[*].

Update the script by:

• Using make package-name instead of make -V pkgname
• Looking for package files with *.pkg instead of *.txz
• Adding a -c option that copies the files instead of linking them[*]

• After 9af130ae8c03 cust_pkgng() cannot be used with a directory

populated by fill_pkg.sh, because it uses a nullfs mount, which doesn't
follow symlinks, therefore the links inside NANO_PACKAGE_DIR will not
work.

PR: 269884
Reviewed by: imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47531