rootfs.py: Drop FAKEROOT support in exec function

rootfs.py: Drop FAKEROOT support in exec function

The _exec_shell_cmd function is used by a couple of other functions that
are ultimatelly called from the create_rootfs function. The latter is
used in image bbclass' do_rootfs which is running using the fakeroot
support in bitbake. This makes the fakeroot support in _exec_shell_cmd
redundant and never actually used.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>