IMAGES_FSTYPES: default to EXT4

IMAGES_FSTYPES: default to EXT4

The following IMAGES_FSTYPES defaulted to ext3:
"vmdk", "vdi", "qcow2", "live", "iso", "hddimg"

This patch changes the default for those IMAGES_FSTYPES to
ext4 in order to bring the images more in line with other BSPs.

Besides improvements in performance and reliability ext4 provides
additional functionality as well (option to turn off the journaling,
dynamic resizing of VDI volumes etc.).

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>