meta-agl-bsp: fix up raspberrypi5 build

Changes:
- Add meta-lts-mixins_u-boot layer to BBLAYERS for raspberrypi5
  to get U-Boot v2024.04.
- Add required configuration to make the newer U-Boot buildable to
  the raspberrypi5 machine configuration.  The tweaks to re-enable
  U-Boot were sourced from the commits in this meta-raspberrypi PR:
  https://github.com/agherzan/meta-raspberrypi/pull/1304
  If those get merged our configuration can be simplified.
- Add required LICENSE_FLAGS_ACCEPTED tweak to raspberrypi5 machine
  configuration (copied from raspberrypi4).
- Remove obsolete Xen tweaks from the raspberrypi4 and raspberrypi5
  machine configurations.

Bug-AGL: SPEC-5144

Change-Id: If02e0cc7baada46f4a9db305d3babe5b4a6784ff
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29908
ci-image-boot-test: Jenkins Job builder account
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>