enable mruby scripting in the nghttpx proxy

This will be needed for (future) handling of TLS certificates.
Apparently, this scripting is the way how it's intended to pass metadata
extracted from clients' TLS certificates to request headers when
proxying requests to the backend server.

A simple buildroot fix is in the `cesnet/2023-09-29` branch.

Change-Id: I56f0481fa24bab2451793c49149cdd50732941c3
diff --git a/configs/czechlight_clearfog_defconfig b/configs/czechlight_clearfog_defconfig
index 20781b2..09151d9 100644
--- a/configs/czechlight_clearfog_defconfig
+++ b/configs/czechlight_clearfog_defconfig
@@ -52,6 +52,7 @@
 BR2_PACKAGE_LIBGPIOD_TOOLS=y
 BR2_PACKAGE_NETOPEER2=y
 BR2_PACKAGE_NETOPEER2_CLI=y
+BR2_PACKAGE_NGHTTP2_MRUBY=y
 BR2_PACKAGE_ELFUTILS=y
 BR2_PACKAGE_LIBUNWIND=y
 BR2_PACKAGE_LINUX_PAM=y
diff --git a/submodules/buildroot b/submodules/buildroot
index acfd253..473753e 160000
--- a/submodules/buildroot
+++ b/submodules/buildroot
@@ -1 +1 @@
-Subproject commit acfd253eadcbcb9453dd66b0b45262c89a899b02
+Subproject commit 473753ed36d0ea2870f6a57d95e713fe8e17b350