package/rpi-userland: add option to install hello-pi examples

rpi-userland comes with a set of demo programs that can be useful to
understand how to program HW-accelerated video/3D/fft/... on the RPi.

They can also be used to validate that the hardware is working properly.

[Thomas: explicitly pass ALL_APPS=OFF when
BR2_PACKAGE_RPI_USERLAND_HELLO is disabled.]

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