This allows for things like btrfs to be used vs just ext4.
The default value of ext4 is kept so there is no functional
change unless VM_ROOTFS_TYPE is set in the inherting recipe.
(From OE-Core rev: df0b217f3df2c36a32e5c4afaec36a28bfc77bbb)
Signed-off-by: Ian Reinhart Geiser <geiseri@geekcentral.pub>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>