wic: use partition size when creating empty partition files

wic: use partition size when creating empty partition files

It seems that prepare_empty_partition_ext() and
prepare_empty_partition_btrfs() got broken in commit
c8669749e37fe865c197c98d5671d9de176ff4dd, thus one could observe the
following backtrace:

Backtrace:
  File "<snip>/poky/scripts/lib/wic/plugins/imager/direct_plugin.py", line 93, in do_create
    creator.create()
  File "<snip>/poky/scripts/lib/wic/imager/baseimager.py", line 159, in create
    self._create()
  File "<snip>/poky/scripts/lib/wic/imager/direct.py", line 290, in _create
    self.bootimg_dir, self.kernel_dir, self.native_sysroot)
  File "<snip>/poky/scripts/lib/wic/partition.py", line 146, in prepare
    method(rootfs, oe_builddir, native_sysroot)
  File "<snip>/poky/scripts/lib/wic/partition.py", line 325, in prepare_empty_partition_ext
    os.ftruncate(sparse.fileno(), rootfs_size * 1024)
NameError: name 'rootfs_size' is not defined

(From OE-Core rev: 0bf686739a5e8a1193f5be5aa60afbf2ea1e5074)

Signed-off-by: Maciej Borzecki <maciej.borzecki@rndity.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>