wic: use native parted

wic: use native parted

Used exec_native_cmd instead of find_binary_path to run parted.
Got rid of find_binary_path as it's not used anywhere else.

There are several tools wic is trying to find not only in sysroot,
but also in host root. Parted is a special as on some distros it's
installed in /usr/sbin, which is not in the user's PATH. This makes
wic to fail with error "External command 'parted' not found, exiting."

[YOCTO #7122]

(From OE-Core rev: 76adf38c0d8e0faf04a5ecb3fcfbe831c85bb81f)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>