Fix mkimage-install problem with external toolchain.

Closes #757.

Make sure that /usr/bin/ exists, before trying to copy mkimage to that
location. When using an external toolchain, /usr/bin is not guarenteed to
exist.

Signed-off-by: Anders Darander <ad@datarespons.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>