rules.mk: add option to download files into a subdirectory

rules.mk: add option to download files into a subdirectory

Some packages download a single raw file or set of raw files
instead of an archive.

The filenames of these download files have names and version numbers
that do not directly correspond to the name and version of the package
as it is defined in its Makefile.

To better organize downloaded files, this adds the option of
creating a subdirectory in the download directory for these files
that can be named with package specific variables.

DL_DIR is now a recursively expanded variable
since the value now depends on whether DL_SUBDIR is defined.

Signed-off-by: Michael Pratt <mcpratt@pm.me>