mpc85xx: retain old compat_version for sysupgrade compatibility

OpenWrt images contain a compat_version, which is used to block upgrades
to newer versions with incompatible configuration, if the configuration
cannot be migrated.

As we maintain our own configuration and all OpenWrt configuration files
are dropped on an upgrade, this upgrade block is not required.

To simplify the upgrade process, retain the old compat_version for the
next sysupgrade release. The compat_version will then be bumped
automatically by the `05_compat-version` board.d script.

Signed-off-by: Fabian Bläse <fabian@blaese.de>

build_patches/openwrt/0052-mpc85xx-retain-old-compat_version.patch

@@ -0,0 +1,17 @@
+From: =?UTF-8?q?Fabian=20Bl=C3=A4se?= <fabian@blaese.de>
+Date: Wed, 24 Jan 2024 16:50:12 +0200
+Subject: [PATCH] mpc85xx: retain old compat_version
+
+diff --git a/target/linux/mpc85xx/image/p1010.mk b/target/linux/mpc85xx/image/p1010.mk
+index c0de19f6c6..5aa580300f 100644
+--- a/target/linux/mpc85xx/image/p1010.mk
++++ b/target/linux/mpc85xx/image/p1010.mk
+@@ -51,8 +51,6 @@ define Device/tplink_tl-wdr4900-v1
+   DEVICE_VENDOR := TP-Link
+   DEVICE_MODEL := TL-WDR4900
+   DEVICE_VARIANT := v1
+-  DEVICE_COMPAT_VERSION := 1.1
+-  DEVICE_COMPAT_MESSAGE := Config cannot be migrated from swconfig to DSA
+   TPLINK_HEADER_VERSION := 1
+   TPLINK_HWID := 0x49000001
+   TPLINK_HWREV := 1