buildtools-tarball: Make buildtools respects host CA certificates

buildtools-tarball: Make buildtools respects host CA certificates

To adapt user network enviroment, buildtools should first try to use
the user configured envs like SSL_CERT_FILE/CURL_CA_BUNDLE/..., if these
envs is not set, then use the auto-detected ca file and ca path, and
finally use the CA certificates in buildtools.

nativesdk-openssl set OPENSSLDIR as "/not/builtin", need set SSL_CERT_FILE/SSL_CERT_DIR to work

nativesdk-curl don't set default ca file, need
SSL_CERT_FILE/SSL_CERT_DIR or CURL_CA_BUNDLE/CURL_CA_PATH to work

nativesdk-git actually use libcurl, and GIT_SSL_CAPATH/GIT_SSL_CAINFO
also works

nativesdk-python3-requests will use cacert.pem under python module certifi by
default, need to set REQUESTS_CA_BUNDLE

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