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/chrony/0001-sys_linux-fix-build-with-older-kernel-headers.patch

@@ -0,0 +1,33 @@
+From 5dab78341bd515692f1b9ff8716a911c3ebf9ee1 Mon Sep 17 00:00:00 2001
+Message-Id: <5dab78341bd515692f1b9ff8716a911c3ebf9ee1.1611839550.git.baruch@tkos.co.il>
+From: Baruch Siach <baruch@tkos.co.il>
+Date: Thu, 28 Jan 2021 15:02:40 +0200
+Subject: [PATCH] sys_linux: fix build with older kernel headers
+
+The renameat2 system call was introduced in kernel version 3.15. Fix
+build against older headers.
+
+Signed-off-by: Baruch Siach <baruch@tkos.co.il>
+---
+Upstream status: commit a2372b0c3abfc85d11c1684c0fb6370cc329e5c4
+
+ sys_linux.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/sys_linux.c b/sys_linux.c
+index 0b24445e173d..202da845edac 100644
+--- a/sys_linux.c
++++ b/sys_linux.c
+@@ -547,7 +547,9 @@ SYS_Linux_EnableSystemCallFilter(int level, SYS_ProcessContext context)
+     SCMP_SYS(readlinkat),
+     SCMP_SYS(rename),
+     SCMP_SYS(renameat),
++#ifdef __NR_renameat2
+     SCMP_SYS(renameat2),
++#endif
+     SCMP_SYS(stat),
+     SCMP_SYS(stat64),
+     SCMP_SYS(statfs),
+-- 
+2.29.2
+