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/ply/0001-configure.ac-fix-error-with-version-string.patch

@@ -0,0 +1,29 @@
+From 684bae6043fa2242a5c6551c3f260c9db0cf7dc8 Mon Sep 17 00:00:00 2001
+From: Andreas Klinger <ak@it-klinger.de>
+Date: Mon, 4 Jan 2021 08:40:50 +0100
+Subject: [PATCH] configure.ac: fix error with version string
+
+When autoreconfiguring there's an error: "configure.ac:6: error: AC_INIT
+should be called with package and version arguments".
+
+Put version string in square brackets to satisfy autoconfigure.
+
+Signed-off-by: Andreas Klinger <ak@it-klinger.de>
+---
+ configure.ac | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index 65387d6..7a0054e 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -1,5 +1,5 @@
+ AC_PREREQ(2.61)
+-AC_INIT(ply, m4_esyscmd_s(git describe --always --dirty),
++AC_INIT(ply, [2.1.1],
+ 	     https://github.com/wkz/ply/issues)
+ 
+ AC_GNU_SOURCE
+-- 
+2.20.1
+