meta-agl-bsp: clean up Raspberry Pi Xen support

meta-agl-bsp: clean up Raspberry Pi Xen support

Changes:
- Rename bbappend for Xen support to work with 5.4 kernel.
- Drop one of the Xen support patches that no longer applies, as it
  seems no longer required due to upstream changes.
- Move Xen kernel configuration fragment use to non-versioned kernel
  bbappend to simplify future upgrades.
- Move ENABLE_UART definition to machine template inc files instead
  of duplicating it in multiple bbappends.  This is more in line with
  how upstream indicates it be used.

Note that these changes have been compile tested with the agl-virt-xen
feature, but the resulting image has not yet been tested with Xen.
This is still an improvement, as agl-virt-xen would have silently not
affected kernel configuration previously.  As well, some of the
reshuffling done here will simplify the kernel upgrade in the next
branch.

Bug-AGL: SPEC-3841

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I022cccef00fcd0a82ceef89d4aab1894004543a3
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26144
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>