[meta-xilinx] IMAGE_FSTYPES default

[Philip Balister]

https://github.com/Xilinx/meta-xilinx/blob/master/conf/machine/include/machine-xilinx-default.inc#L4

sets

IMAGE_FSTYPES ?= "cpio tar.gz ext2 ext2.gz.u-boot cpio.gz.u-boot"

This causes problems for people building large images. Image build take
a long time and you can easily run out of disk space. I've been
overriding this in local.conf, but maybe a smaller set of default images
would be helpful.

Philip