buildtools-tarball: move setting of envvars to respective envfile
* make git,curl,python3-requests align with openssl, move the setting of
envvars into respective envfile
* for environment.d-openssl.sh, also check if ca-certificates.crt exist
before export envvars
Signed-off-by: Changqing Li <changqing.li@windriver.com> Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>