qt5base: fix mkspecs for wayland-scanner.

This patch from upstream fixes the copy of the header files generated by
wayland-scanner (needed to build QtWayland).
qt5base: force EGLFS_PLATFORM_HOOKS_LIBS for Raspberry Pi.

EGLFS in Qt 5.2.0 for Raspberry Pi does not compile when rpi-userland
5cecd5ae71540a3c11270924251103b3b5c8513a is used, because of undefined
references to some vc_dispmanx_* symbols.

Linking against libvchostif fixes this issue. Normally, -lvchostif
should be added to EGLFS_PLATFORM_HOOKS_LIBS in
mkspecs/devices/linux-rasp-pi-g++/qmake.conf, but Buildroot overrides
the device selection in qt5base.mk. So the correct value for
EGLFS_PLATFORM_HOOKS_LIBS is set from this Makefile.
qt5base: make qRound(double d) static to avoid SIGILL.

Running Qt5 5.2.0 examples on Rasperry Pi ends up with a SIGILL:
https://bugreports.qt-project.org/browse/QTBUG-30440.
This patch implements only a workaround suggested by T. Macieira.
qt5wayland: new package.

QtWayland is a Qt5 module that wraps the functionality of Wayland.
board: add elebihan demo-qtwayland.

New configuration files are available to build a kernel and file system to
showcase Wayland support for Qt 5, one for QEMU x86 and the other for
Raspberry Pi.
host-gdb: enable Python only if host-python exists.

When building host-gdb, Python support is enabled if a Python
interpreter is found in the path. If host-python has been built (as a
dependency of another package), it will be used.

But at runtime, the cross-gdb will execute the Python interpreter
available on the build system, which may result with the error:

AttributeError: 'module' object has no attribute 'pydebug'.

As there is no BR2_HOST_python symbol, the Makefile checks for the
existence of /path/to/host/usr/bin/python and forces its use if true.