package/rpi-firmware: select BR2_LINUX_KERNEL_DTB_OVERLAY_SUPPORT when needed

When BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS is enabled, and the
DTBs are built by Linux (i.e BR2_LINUX_KERNEL_DTS_SUPPORT is enabled),
these DTBs should be built with the -@ Device Tree compiler option, so
that they can be used together with DTB overlays. So let's select
BR2_LINUX_KERNEL_DTB_OVERLAY_SUPPORT in this situation.

Fixes:

  https://bugs.busybox.net/show_bug.cgi?id=12831



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