Add support for the uuu flasher

This adds flasher support for uuu, a utility used by NXP devices for flashing images.

The flasher expects a uuu command list script to be installed in the device rootfs at /usr/share/uuu/flash_script.lst.

The flash_method in deviceinfo to enable it is 'uuu'.

It can be used with pmbootstrap flasher flash_rootfs after the rootfs image has been built.