package/matchbox: fix option prompts

Use the package names instead of some fancy human-readable strings. ;-)

Note: the matchbox package is really named matchbox-window-manager by
upstream. However:
  - it already is in a section of the menu title 'X window managers'
  - users are probably only looking for 'matchbox'
  - keeping just 'matchbox' keeps it in sync with the Kconfig variable,
    and allows us to not handle a legacy entry (see a previous commit)
  - matchbox-window-manager is a bit long. ;-)

So, in this case, the short 'matchbox' prompt is used instead of the
real upstream name.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>