package/opencv: depends on GUI toolkits, rather than select them

Currently, we only support either Qt4 or gtk2, but OpenCV also
additionally supports Qt5 or gtk3, making for a choice of four
toolkits, one of: Qt4, Qt5, gtk3, gtk2 (and obviously, none).

Since Buildroot does not support coexistence of Qt4 and Qt5, we
can no longer select one and depend on the other, like so:

    config BR2_PKG_OCV_WITH_QT4
        bool "Qt4"
        depends on !BR2_PKG_QT5
        select BR2_PKG_QT

    config BR2_PKG_OCV_WITH_QT5
        bool "Qt5"
        depends on !BR2_PKG_QT
        select BR2_PKG_QT5

otherwise, we'd get a circular dependency chain in Kconfig, which would
complain with:

  package/opencv/Config.in:57:error: recursive dependency detected!
  package/opencv/Config.in:57:    choice <choice> contains symbol BR2_PKG_OCV_WITH_QT5
  package/opencv/Config.in:111:   symbol BR2_PKG_OCV_WITH_QT5 depends on BR2_PKG_QT
  package/qt/Config.in:5: symbol BR2_PKG_QT is selected by BR2_PKG_OCV_WITH_QT
  package/opencv/Config.in:98:    symbol BR2_PKG_OCV_WITH_QT is part of choice <choice>

Instead, we need to depend on either Qt version.

So, to have a consistent choice, we make all support for GUI toolkits
actually depend on the toolkit, rather than select it.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: split-out the switch-selects-to-depends hunk
 from the add-qt5 hunk]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>