rpi-firmware: allow disabling installation of binary DTBs

The binary .dtb files are not suitable for everyone as they are
kernel-version specific.

Reintroduce BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS option.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
[yann.morin.1998@free.fr: don't install DTBs if kernel builds
 its own; fix default; rephrase help text]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>