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/daemon/0001-fix-musl.patch

@@ -8,11 +8,11 @@ Signed-off-by: Baruch Siach <baruch@tkos.co.il>
 diff -Nuar daemon-0.6.4-orig/daemon.c daemon-0.6.4/daemon.c
 --- daemon-0.6.4-orig/daemon.c	2010-06-12 16:37:00.000000000 +0300
 +++ daemon-0.6.4/daemon.c	2017-05-01 22:02:17.721413468 +0300
-@@ -665,6 +665,7 @@
+@@ -864,6 +864,7 @@
  #ifdef _RESTORE_POSIX_SOURCE
  #define _POSIX_SOURCE
  #endif
 +#include <sys/ttydefaults.h>
+ #include <dirent.h>
  #include <sys/wait.h>
  #include <sys/stat.h>
- #include <sys/select.h>