Makefile should compress resulting initrd.img

[TJC]

The default boot partition is quite small - often just 16MB.

The initrd can be compressed, which easily halves the size with gzip, or goes even smaller with other compression methods.

Many people probably don't realise this, and are likely to accidentally create too-large initrds. As such, the Makefile should add a "gzip initrd.img" for an easy win.

(I've tested this myself, and just booted a Rock using a compressed initrd)