curl: only set CA bundle in target build

curl: only set CA bundle in target build

In native/nativesdk builds, sysconfdir refers to a recipe sysroot
directory, which will disappear once the workdir is cleaned up, breaking
libcurl's HTTPS connections.

By simply not setting --with-ca-bundle at all in non-target builds, curl
defaults to the host system's CA certificates, which is desirable anyways
to allow builds in environments that require local CA certificates.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>