vexpress64: fvp: add distro_boot support

vexpress64: fvp: add distro_boot support

So far the FVP model just supports booting through semihosting, so by
loading files from the host the model is running on. This allows for
quick booting of new kernels (or replacing DTBs), but prevents more
featureful boots like using UEFI.

Enable the distro_boot feature, and provide a list of possible boot
sources that U-Boot should check:
- For backwards compatibility we start with semihosting, which gets its
  commands migrated from CONFIG_BOOTCOMMAND into the distro_boot
  infrastructure. This is also slightly tweaked to fail graceful in case
  the required files could not be found.
- Next we try to use a user provided script, that could be easily
  placed into memory using the model command line.
- Since we gained virtio support with the enablement of OF_CONTROL,
  let's check virtio block devices next. This is where UEFI boot can
  be easily used, for instance by providing a distro installer .iso
  file through virtio-blk.
- Networking is now provided by virtio as well, so enable the default
  PXE and DHCP boot flows, mostly because we can.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>