Add a Buildroot 'device' to ease cross-compilation

Qt5 has a mechanism to support "device" profiles, so that people can
specify the compiler, compiler flags and so on for a specific device.

We leverage this mechanism in the Buildroot packaging of qt5 to
simplify cross-compilation: we have our own "device" definition, which
allows us to easily pass the cross-compiler paths and flags from our
qt5.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/mkspecs/devices/linux-buildroot-g++/qmake.conf
===================================================================
--- /dev/null
+++ b/mkspecs/devices/linux-buildroot-g++/qmake.conf
@@ -0,0 +1,16 @@
+include(../common/linux_device_pre.conf)
+
+# modifications to g++-unix.conf
+QMAKE_CC                = $${BR_CCACHE} $${CROSS_COMPILE}gcc
+QMAKE_CXX               = $${BR_CCACHE} $${CROSS_COMPILE}g++
+
+#modifications to gcc-base.conf
+QMAKE_CFLAGS           += $${BR_COMPILER_CFLAGS}
+QMAKE_CXXFLAGS         += $${BR_COMPILER_CXXFLAGS}
+QMAKE_CXXFLAGS_RELEASE += -O3
+CONFIG                 += nostrip
+
+QMAKE_LIBS             += -lrt -lpthread -ldl
+
+include(../common/linux_device_post.conf)
+load(qt_config)
Index: b/mkspecs/devices/linux-buildroot-g++/qplatformdefs.h
===================================================================
--- /dev/null
+++ b/mkspecs/devices/linux-buildroot-g++/qplatformdefs.h
@@ -0,0 +1 @@
+#include "../../linux-g++/qplatformdefs.h"