Clean up Raspberry Pi Xen kernel configuration

At the moment the kernel patches and configuration fragment for the contributed Xen support are in a bbappend specific to the 4.19 kernel.  In Koi the 5.4 kernel is being used so those are ineffective, so some form of remediation is required.  As well, the changes that were made with respect to the ENABLE_UART variable are now stale with respect to how upstream meta-raspberrypi expects the variable to be used (it needs to be globally defined to ensure it gets used everywhere), and testing indicates the resulting value in the default non-Xen case broke kernel serial console output.