curl: add PACKAGECONFIG option to use libssh2

curl: add PACKAGECONFIG option to use libssh2

The user can enable libssh2 via conf/local.conf or custom distro
configuration, this will pull in libssh2, which is not used by default.

For example, a curl_x.y.z.bbappend file containing the following line:
PACKAGECONFIG += "libssh2"

(From OE-Core rev: d425e005d274cac0ef7160f53c41bda175444f69)

Signed-off-by: Fabrice Coulon <fabrice.coulon@axis.com>
Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>