fix error in nginx.mk by renaming NGINX_CONF_OPT to NGINX_CONF_OPTS

error message was:

`package/nginx/nginx.mk:227: *** Package error: use NGINX_CONF_OPTS instead of NGINX_CONF_OPT. Please fix your .mk file.  Stop.`
fix missing git dependency when running `make build` in example directory

error message was:

`You must install 'git' on your build machine`