zram: add package to use compressed ram disk for swap

The busybox size is increased by 3.2KByte uncompressed on bcm47xx with
this commit.

This zram-swap automatically creates a zram device, a swap partition on
it and make the kernel swap pages to it.

Thank you Bastian Bittorf <bittorf@bluebottle.com> for idea and the
script.

SVN-Revision: 35025