curl: Use ca-bundle for all TLS libraries.

curl: Use ca-bundle for all TLS libraries.

It simplifies the Makefile a bit. In addition, using ca-bundle
saves some space as well.

It also fixes an issue with at least transmission, which has a dependency
on ca-bundle, but currently libcurl with OpenSSL or GnuTLS cause it not
to work.

This has been tested on mt7621 with OpenSSL and GnuTLS just by running
'curl https://www.google.com' and seeing if there's a verify error.
The rest are already using ca-bundle and therefore work fine.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Tested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(backported from f97946c49680a5fe713d0e2caaf072789f70e68d)