kernel-development: Note that the workflow also applies to u-boot

OpenBMC's u-boot fork has historically been a fork of ASPEED's fork.
Experience suggests this is not a sustainable way to maintain platform
support. Instead, require changes go upstream and be backported into
OpenBMC's u-boot fork, in the same manner as kernel patches.

Change-Id: I37d57bcf896aff1439966acc66816a59e162de19
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/kernel-development.md b/kernel-development.md
index 43cfd54..30c1ef2 100644
--- a/kernel-development.md
+++ b/kernel-development.md
@@ -1,5 +1,10 @@
 # OpenBMC kernel development
 
+> [!IMPORTANT]
+>
+> The kernel development philosphy described below also applies to OpenBMC's
+> u-boot fork.
+
 The OpenBMC project maintains a kernel tree for use by the project. The tree's
 general development policy is that code must be upstream first. This is strongly
 desirable but not a hard requirement, and exceptions may be made on a