patches/openwrt: enable earlyprink for easier UART debugging on Omnia

Add work in progress patch, which needs to be polished, so we can send
this to upstream later.

Fixes: #347

patches/openwrt/5.15-kernel-configuration/0021-mvebu-cortexa9-enable-earlyprintk.patch

+From 56079e545ce4734ee78ac5f91923e95503e996dd Mon Sep 17 00:00:00 2001
+From: Josef Schlehofer <pepe.schlehofer@gmail.com>
+Date: Wed, 27 Jul 2022 13:32:49 +0200
+Subject: [PATCH] mvebu/cortexa9: enable earlyprintk
+
+---
+ target/linux/mvebu/cortexa9/config-5.15 | 5 +++++
+ 1 file changed, 5 insertions(+)
+
+diff --git a/target/linux/mvebu/cortexa9/config-5.15 b/target/linux/mvebu/cortexa9/config-5.15
+index b737c44d42..b1f2452d52 100644
+--- a/target/linux/mvebu/cortexa9/config-5.15
++++ b/target/linux/mvebu/cortexa9/config-5.15
+@@ -1,2 +1,7 @@
+ CONFIG_PHY_MVEBU_A38X_COMPHY=y
+ CONFIG_RTC_DRV_MV=y
++
++#Earlyprintk (https://gitlab.nic.cz/turris/os/build/-/issues/347)
++CONFIG_CMDLINE="earlyprintk console=ttyS0,115200"
++CONFIG_CMDLINE_FROM_BOOTLOADER=y
++CONFIG_DEBUG_MVEBU_UART0_ALTERNATE=y
+-- 
+2.34.1
+