Add start_package_http target

Run simple HTTP server  in fs/output/xpackage to install
freshly build packages directly from SDK/fs without any copying or SD
card swap.

The HTTP server is a sample python package bundled with most of python
releases.

To install fresh build packages from your SDK root.

On SDK host:

0. build your favorite package

1. Run server 'make  start_package_http' in SDK root dir

On virt2real:

0. Update virt2real's /etc/opkg/opkg.conf with the following line
src "all" http://<your-machine-IP-address>:8000
This need to be done just once.

1. Update package list with 'opkg-cl update'

2. Install package with 'opkg-cl install <your-favorite-package-name>'