qt5: Add an option to select default platform

Whithout this path, default platform is automaticaly set (generally
set to "eglfs" as defined in
qt5base/mkspecs/devices/common/linux_device_pre.conf:1). This choice
is not always what the user would like. Thus, user have to manually
appends "-platform <BACKEND>" to command line when running any qt5
application.

This patch allows user to choose default platform explicitly.

[Thomas: slightly improve Config.in help text, from the suggestions
given by Yann E. Morin.]

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/qt5/qt5base/Config.in

@@ -144,6 +144,17 @@ config BR2_PACKAGE_QT5BASE_EGLFS
 comment "eglfs backend available if OpenGLES and EGL are enabled"
 	depends on !BR2_PACKAGE_HAS_LIBEGL || !BR2_PACKAGE_HAS_LIBGLES
 
+config BR2_PACKAGE_QT5BASE_DEFAULT_QPA
+	string "Default graphical platform"
+	help
+	  Choose the default platform abstraction to use for graphical
+	  applications (e.g xcb, linuxfb, eglfs, ...). If this is empty, the
+	  default for your architecture will be used (usually this is eglfs).
+
+	  You can get a list of supported platforms by running a Qt application
+	  with the option "-platform help" on your target. You can choose a
+	  different platform at runtime with the -platform option.
+
 config BR2_PACKAGE_QT5BASE_PRINTSUPPORT
 	bool "print support module"
 	select BR2_PACKAGE_QT5BASE_WIDGETS

package/qt5/qt5base/qt5base.mk

@@ -96,6 +96,9 @@ else
 QT5BASE_CONFIGURE_OPTS += -no-xcb
 endif
 
+QT5BASE_DEFAULT_QPA = $(call qstrip,$(BR2_PACKAGE_QT5BASE_DEFAULT_QPA))
+QT5BASE_CONFIGURE_OPTS += $(if $(QT5BASE_DEFAULT_QPA),-qpa $(QT5BASE_DEFAULT_QPA))
+
 ifeq ($(BR2_PACKAGE_QT5BASE_EGLFS),y)
 QT5BASE_CONFIGURE_OPTS += -opengl es2 -eglfs
 QT5BASE_DEPENDENCIES   += libgles libegl