wic: Use os.rename instead of bb.utils.rename

wic: Use os.rename instead of bb.utils.rename

This is not invoked with bitbake context as a result bb.utils is not
visible when this function is called during image creation and builds
fail e.g.

NameError: name 'bb' is not defined

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Joshua Watt <JPEWhacker@gmail.com>
Cc: Devendra Tewari <devendra.tewari@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>