wic: Allow exec_native_cmd to run HOSTTOOLS

This allows programs from HOSTTOOLS (e.g. 'install', 'rm', 'mv', etc) to
be more easily executed by wic. Without this change only programs from
an actual *-native recipe built by bitbake can be executed by wic.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>