webos: Change default IMAGE_FSTYPES setting

* bitbake.conf sets tar.gz as sensible default, but we cannot use it
  because when building for MACHINE different from qemux86, vmdk
  conditional sets IMAGE_FSTYPES to empty string and
  IMAGE_FSTYPES ?= "tar.gz" from bitbake.conf is not used

* remove ext3 from default, because it's really big and should be included
  only for MACHINEs which use it

* using tar.bz2 without pbzip2 or lbzip2 installed on host machine is slow
  and resulting images have almost the same size, tar.gz on the other hand
  is using pigz-native provided by OE without need for any additional setup

* IMAGE_FSTYPES="tar.gz ext3 vmdk" is used for qemux86* MACHINEs
  IMAGE_FSTYPES="tar.gz ext3" is used for other qemu* MACHINEs

[GF-2578] Change default IMAGE_TYPES
[QUES-280] Can do_rootfs be sped up?

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: Idf4ee9b4d5bb69cde6ac5b19f6fad547671f34db
Reviewed-on: https://g2g.palm.com/1746
Reviewed-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Build Verification
Tested-by: Martin Jansa <martin.jansa@palm.com>
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>