rpi-firmware: move version maintenance to system

This moves the rpi-firmware version selection to the Nerves system. This
makes it much easier to update kernels since you no longer need to
update `nerves_system_br` to add an option to select the new
rpi-firmware.

All Raspberry Pi Nerves systems will need to be updated to add something
like the following line to their `nerves_defconfig`:

```
BR2_PACKAGE_RPI_FIRMWARE_CUSTOM_VERSION="1.20200902"
```

If this isn't done, the build will fail with a long error message.

patches/buildroot/0010-rpi-firmware-support-custom-version-selection.patch

@@ -0,0 +1,61 @@
+From 442fa4e2689f35c2504c0a9d6b6afb797838d421 Mon Sep 17 00:00:00 2001
+From: Frank Hunleth <fhunleth@troodon-software.com>
+Date: Mon, 8 Jul 2019 10:44:12 -0400
+Subject: [PATCH] rpi-firmware: support custom version selection
+
+Since the rpi-firmware supplies firmware binaries that have the
+potential to depend on kernel drivers, we've matched kernel versions to
+rpi-firmware versions. In order to simplify using different Linux kernel
+versions, this adds an option to set the tag or hash to use. For
+example, if you're using the Raspberry Pi Linux kernel tagged
+`1.20200902`, you can now use the rpi-firmware version that was tagged
+`1.20200902`.
+---
+ package/rpi-firmware/Config.in       |  5 +++++
+ package/rpi-firmware/rpi-firmware.mk | 14 +++++++++++++-
+ 2 files changed, 18 insertions(+), 1 deletion(-)
+
+diff --git a/package/rpi-firmware/Config.in b/package/rpi-firmware/Config.in
+index f5a25c5680..550acda3ad 100644
+--- a/package/rpi-firmware/Config.in
++++ b/package/rpi-firmware/Config.in
+@@ -29,6 +29,11 @@ config BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI4
+ 	  The Raspberry Pi 4 files
+ endchoice
+
++config BR2_PACKAGE_RPI_FIRMWARE_CUSTOM_VERSION
++	string "custom version"
++	help
++	  If specified, use this tag or git hash instead of the default
++
+ choice
+ 	bool "Firmware to boot"
+ 	default BR2_PACKAGE_RPI_FIRMWARE_DEFAULT
+diff --git a/package/rpi-firmware/rpi-firmware.mk b/package/rpi-firmware/rpi-firmware.mk
+index 8aa855d0ef..235b13b905 100644
+--- a/package/rpi-firmware/rpi-firmware.mk
++++ b/package/rpi-firmware/rpi-firmware.mk
+@@ -4,7 +4,19 @@
+ #
+ ################################################################################
+
+-RPI_FIRMWARE_VERSION = 542aceb30364db3ce87d532ea90451f46bbb84e9
++ifeq ($(call qstrip,$(BR2_PACKAGE_RPI_FIRMWARE_CUSTOM_VERSION)),)
++ifeq ($(BR2_PACKAGE_RPI_FIRMWARE),y)
++$(error The Raspberry Pi firmware version is no longer derived automatically.\
++    Please set BR2_PACKAGE_RPI_FIRMWARE_CUSTOM_VERSION in your nerves_defconfig\
++    to a version that is compatible with the Linux kernel that you are using.\
++    For example, if using a tagged Raspberry Pi kernel, there usually is a \
++    corresponding tag at https://github.com/raspberrypi/firmware)
++endif
++else
++RPI_FIRMWARE_VERSION = $(call qstrip,$(BR2_PACKAGE_RPI_FIRMWARE_CUSTOM_VERSION))
++BR_NO_CHECK_HASH_FOR += $(RPI_FIRMWARE_SOURCE)
++endif
++
+ RPI_FIRMWARE_SITE = $(call github,raspberrypi,firmware,$(RPI_FIRMWARE_VERSION))
+ RPI_FIRMWARE_LICENSE = BSD-3-Clause
+ RPI_FIRMWARE_LICENSE_FILES = boot/LICENCE.broadcom
+-- 
+2.25.1
+