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

(From OE-Core rev: 5f4fd544d3df7365224599c9efdce4e545f51d5e)

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-devtools/python/python3-requests/environment.d-python3-requests.sh

@@ -0,0 +1,3 @@
+if [ -e "${OECORE_NATIVE_SYSROOT}/etc/ssl/certs/ca-certificates.crt" ];then
+    export REQUESTS_CA_BUNDLE="${OECORE_NATIVE_SYSROOT}/etc/ssl/certs/ca-certificates.crt"
+fi