package/weston: add selection menu for default backend

package/weston: add selection menu for default backend

With the upcoming bump to weston 8, the meaning for the "native backend"
has been removed, and replaced by a new option to set the "default
backend". However, the way we handle the "native backend" option
nowadays is very semantically similar to the upcoming "default backend"
option.

However, as of today, when more than one backend are enabled, the last
one defined in the .mk wins, which is not obvious when looking at the
menuconfig order...

So, we introduce a choice to select the default backend. That enforces
the backend is enabed, leaving the others as additional backends.

It is to be noted that the RDP backend can't be selected as the native
(soon default) backend, so it does not get an entry in the choice.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[yann.morin.1998@free.fr:
  - write a commit log
  - merge short lines
  - rename optons (no _BACKEND)
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>