board/raspberrypi: handle dtb overlays for all variants

Most rpi defconfigs use dtb overlays, but not rpi0 / rpi2 - Making it harder
to use overlays on those boards as the genimage files have to be tweaked.

To fix this, create the rpi-firmware/overlays directory in the post-build
script if needed and unconditionally include it in the genimage files so
rpi0/rpi2 works consistently with the other variants.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 6021f3678b44d43da7057e3135da842cde1ffa05)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/raspberrypi/genimage-raspberrypi0.cfg

@@ -7,6 +7,7 @@ image boot.vfat {
 			"rpi-firmware/config.txt",
 			"rpi-firmware/fixup.dat",
 			"rpi-firmware/start.elf",
+			"rpi-firmware/overlays",
 			"zImage"
 		}
 	}

board/raspberrypi/genimage-raspberrypi2.cfg

@@ -7,6 +7,7 @@ image boot.vfat {
 			"rpi-firmware/config.txt",
 			"rpi-firmware/fixup.dat",
 			"rpi-firmware/start.elf",
+			"rpi-firmware/overlays",
 			"zImage"
 		}
 	}

board/raspberrypi/post-build.sh

@@ -9,3 +9,6 @@ if [ -e ${TARGET_DIR}/etc/inittab ]; then
 	sed -i '/GENERIC_SERIAL/a\
 tty1::respawn:/sbin/getty -L  tty1 0 vt100 # HDMI console' ${TARGET_DIR}/etc/inittab
 fi
+
+# exnsure overlays exists for genimage
+mkdir -p "${BINARIES_DIR}/rpi-firmware/overlays"