vexpress64: config header: unify environment definition

vexpress64: config header: unify environment definition

The definition of the standard environment variables (kernel_addr_r and
friends) has been improved lately for the FVP model, but the Juno board
is still using some custom scheme.
Since we need to extend this to a third board soon, let's unify the
definition:
- Define the Juno addresses in the same generic way we do for the FVP
  model, and move the actual variable setting out of the board #ifdef's.
- Add the missing addresses for a PXE file and a boot script.
- Cleanup some stale comments on the way.

As the FVP model doesn't have support for distro_boot quite yet, add
a dummy definition for now, to be replaced with the real thing later.

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