package/weston: add configuration options for weston shells

package/weston: add configuration options for weston shells

Enable selection of used weston shells. By default all available
shells are enabled to keep the old behavior. The new configuration
options enable the user to select them individually.

Signed-off-by: Martin Elshuber <martin.elshuber@theobroma-systems.com>
[yann.morin.1998@free.fr:
  - ensure at least one shell is enabled; desktop arbitrarily chosen
  - s/BR2_PACKAGE_WESTON_SHELL_SELECTED/BR2_PACKAGE_WESTON_HAS_SHELL/
  - reword associated comment
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>