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 +1,43 @@
+MAKEFILE_GENERATOR      = UNIX
+CONFIG                  += incremental gdb_dwarf_index
+QMAKE_INCREMENTAL_STYLE = sublib
+
+include(../../common/linux.conf)
+include(../../common/gcc-base-unix.conf)
+include(../../common/g++-unix.conf)
+
+load(device_config)
+
+QT_QPA_DEFAULT_PLATFORM = eglfs
+
+BUILDROOT_CROSS_COMPILE =
+BUILDROOT_COMPILER_CFLAGS =
+BUILDROOT_COMPILER_CXXFLAGS =
+BUILDROOT_INCLUDE_PATH =
+
+# modifications to g++.conf
+QMAKE_CC                = $${BUILDROOT_CROSS_COMPILE}gcc
+QMAKE_CXX               = $${BUILDROOT_CROSS_COMPILE}g++
+QMAKE_LINK              = $${QMAKE_CXX}
+QMAKE_LINK_SHLIB        = $${QMAKE_CXX}
+
+# modifications to linux.conf
+QMAKE_AR                = $${BUILDROOT_CROSS_COMPILE}ar cqs
+QMAKE_OBJCOPY           = $${BUILDROOT_CROSS_COMPILE}objcopy
+QMAKE_STRIP             = $${BUILDROOT_CROSS_COMPILE}strip
+
+#modifications to gcc-base.conf
+QMAKE_CFLAGS           += $${BUILDROOT_COMPILER_CFLAGS}
+QMAKE_CXXFLAGS         += $${BUILDROOT_COMPILER_CXXFLAGS}
+QMAKE_CXXFLAGS_RELEASE += -O3
+INCLUDEPATH            += $${BUILDROOT_INCLUDE_PATH}
+
+QMAKE_LIBS             += -lrt -lpthread -ldl
+
+# device specific glue code
+EGLFS_PLATFORM_HOOKS_SOURCES =
+
+# Sanity check
+deviceSanityCheckCompiler()
+
+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"