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.


git-svn-id: svn://svn.openwrt.org/openwrt@35025 3c298f89-4303-0410-b956-a3cf2f4a3e73