Add a new xen-rpi-u-boot-scr recipe to PROVIDE u-boot-default-script
instead of using a bbappend to rpi-u-boot-scr.

This allows the new recipe to be selected by indicating it as
PREFERRED_PROVIDER of u-boot-default-script, and then do that in the
Xen on Raspberry Pi 4 config file, xen-raspberrypi4-64.inc.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>