Bump Buildroot to 2021.02-rc3 (#1260)

* Rebase patches to Buildroot 2021.02-rc3

* Update Buildroot to 2021.02-rc3

* Declare Kernel headers to be Linux version 5.10 (since they are, and new Buildroot knows about 5.10)

buildroot/package/botan/0002-Avoid-empty-l.patch

@@ -0,0 +1,25 @@
+From 4b2225ea54988e09b649768848b3c14b43e064de Mon Sep 17 00:00:00 2001
+From: Jack Lloyd <jack@randombit.net>
+Date: Mon, 15 Feb 2021 15:11:12 -0500
+Subject: [PATCH] Avoid empty -l
+
+[Retrieved from:
+https://github.com/randombit/botan/commit/4b2225ea54988e09b649768848b3c14b43e064de]
+Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
+---
+ configure.py | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.py b/configure.py
+index 865a6c8e62..a79f672eca 100755
+--- a/configure.py
++++ b/configure.py
+@@ -2027,7 +2027,7 @@ def extra_libs(libs, cc):
+         if libs is None:
+             return ''
+ 
+-        return ' '.join([(cc.add_lib_option % lib) for lib in libs.split(',')])
++        return ' '.join([(cc.add_lib_option % lib) for lib in libs.split(',') if lib != ''])
+ 
+     variables = {
+         'version_major':  Version.major(),