Add infrastructure to download tarballs

Instead of storing the various tarballs in the ONIE project modify the
build system to download (and cache) the tarballs at compile time.

A new directory is created under the build directory:

  build/download

The various components' tarballs are stored in build/download.

A top level "make clean" will *not* delete the contents of
build/download.  To also remove the download directory use
"make clean-all".

To download all the files and stop use "make download".

TO only delete the downloaded files use "make download-clean".

Two new helper scripts are also added:

- fetch-package -- if the local file does not exist, download it using
                   a list of base URLs

- extract-package -- untar the file in a specified location

Testing:

1. Rebuilt all supported platforms
2. Installed ONIE and exercised features
busybox: use new package download infrastructure

Download busybox source code at compile time.
dropbear: use new package download infrastructure

Download dropbear source code at compile time.
e2fsprogs: use new package download infrastructure

Download e2fsprogs source code at compile time.
kernel: use new package download infrastructure

Download kernel source code at compile time.
lzo: use new package download infrastructure

Download lzo source code at compile time.
mtd-utils: use new package download infrastructure

Download mtd-utils at compile time.

This package has a small wrinkle as the tarball is downloaded via
gitweb.

The 1.5.0 version is actually specified via its git hash, ca39eb1,
which creates a tarball of the same name.  After download, a symlink
is made to the more readable mtd-utils-1.5.0 name.
lzo: use new package download infrastructure

Download lzo source code at compile time.
uclibc: use new package download infrastructure

Download uclibc source code at compile time
zlib: use new package download infrastructure

Download zlib source code at compile time.