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/libmdbx/Config.in

@@ -0,0 +1,45 @@
+config BR2_PACKAGE_LIBMDBX
+	bool "libmdbx"
+	depends on BR2_USE_MMU
+	depends on BR2_TOOLCHAIN_HAS_SYNC_4
+	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_4
+	help
+	  One of the fastest compact key-value ACID database
+	  without WAL. libmdbx has a specific set of properties
+	  and capabilities, focused on creating unique lightweight
+	  solutions.
+
+	  libmdbx surpasses the legendary LMDB in terms of
+	  reliability, features and performance.
+
+	  https://github.com/erthink/libmdbx
+
+if BR2_PACKAGE_LIBMDBX
+
+config BR2_PACKAGE_LIBMDBX_TOOLS
+	bool "install tools"
+	help
+	  Install libmdbx tools for checking, dump, restore
+	  and show statistics of databases.
+
+config BR2_PACKAGE_LIBMDBX_CXX
+	bool "C++ API"
+	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+	depends on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735
+	help
+	  Enable modern C++11/14/17/20 API for libmdbx.
+
+comment "libmdbx C++ support needs a toolchain w/ C++11, gcc >= 4.8 w/o bug#64735"
+	depends on !BR2_INSTALL_LIBSTDCPP || \
+		!BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 || \
+		BR2_TOOLCHAIN_HAS_GCC_BUG_64735
+
+endif
+
+comment "libmdbx needs MMU, a toolchain w/ threads, gcc >= 4.4 w/ 4-byte atomics"
+	depends on BR2_USE_MMU
+	depends on !BR2_TOOLCHAIN_HAS_THREADS || \
+		!BR2_TOOLCHAIN_HAS_SYNC_4 || \
+		!BR2_TOOLCHAIN_GCC_AT_LEAST_4_4