perl: add PACKAGECONFIG for db

perl: add PACKAGECONFIG for db

perl-cross makes it easy to cleanly disable the use of Berkeley DB if desired,
so add a PACKAGECONFIG for this.

More of the dependencies (such as gdbm) can be controlled in the same way, but
Berkeley DB is a prime contender: it's a large library which we have to ship an
old release for due to licensing issues, so there are users who may want to
strip it out of builds entirely.

(From OE-Core rev: e3860098dadd0b1db97fb0414aae6abfc234b245)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>