Upgrade buildroot to 2023.05 (from 2021.08.2), kernel is upgraded to 6.3 (from 5.13.19).

system/Config.in

@@ -78,7 +78,6 @@ endchoice # Passwd encoding
 
 config BR2_TARGET_GENERIC_PASSWD_METHOD
 	string
-	default "md5"       if BR2_TARGET_GENERIC_PASSWD_MD5
 	default "sha-256"   if BR2_TARGET_GENERIC_PASSWD_SHA256
 	default "sha-512"   if BR2_TARGET_GENERIC_PASSWD_SHA512
 
@@ -125,22 +124,26 @@ config BR2_INIT_SYSTEMD
 	depends on BR2_TOOLCHAIN_USES_GLIBC
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	depends on BR2_TOOLCHAIN_HAS_THREADS
-	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_15
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_5
 	depends on BR2_HOST_GCC_AT_LEAST_5
 	select BR2_ROOTFS_MERGED_USR
 	select BR2_PACKAGE_SYSTEMD
 	select BR2_PACKAGE_SKELETON_INIT_SYSTEMD if BR2_ROOTFS_SKELETON_DEFAULT
 
-comment "systemd needs a glibc toolchain w/ SSP, headers >= 3.10, host and target gcc >= 5"
+comment "systemd needs a glibc toolchain w/ SSP, headers >= 3.15, host and target gcc >= 5"
 	depends on BR2_PACKAGE_SYSTEMD_ARCH_SUPPORTS
 	depends on BR2_USE_MMU
 	depends on !BR2_TOOLCHAIN_USES_GLIBC || \
 		!BR2_TOOLCHAIN_HAS_SSP || \
-		!BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10 || \
+		!BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_15 || \
 		!BR2_TOOLCHAIN_GCC_AT_LEAST_5 || \
 		!BR2_HOST_GCC_AT_LEAST_5
 
+comment "systemd highly recommends Linux >= 4.15"
+	depends on BR2_INIT_SYSTEMD
+	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_15
+
 config BR2_INIT_NONE
 	bool "None"
 	select BR2_PACKAGE_SKELETON_INIT_NONE if BR2_ROOTFS_SKELETON_DEFAULT
@@ -154,6 +157,57 @@ source "$BR2_BASE_DIR/.br2-external.in.init"
 
 endchoice
 
+if BR2_INIT_SYSTEMD
+
+config BR2_INIT_SYSTEMD_VAR_FACTORY
+	bool "build a factory to populate a tmpfs on /var"
+	default y  # legacy
+	depends on !BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW
+	help
+	  Build a factory of the content of /var as installed by
+	  packages, mount a tmpfs on /var at runtime, so that
+	  systemd-tmpfiles can populate it from the factory.
+
+	  This may help on a read-only rootfs.
+
+	  It probably does not play very well with triggering a call
+	  to systemd-tmpfiles at build time (below).
+
+	  Note: Buildroot mounts a tmpfs on /var to at least make the
+	  system bootable out of the box; mounting a filesystem from
+	  actual storage is left to the integration, as it is too
+	  specific and may need preparatory work like partitionning a
+	  device and/or formatting a filesystem first, so that falls
+	  out of the scope of Buildroot.
+
+	  To use persistent storage, provide a systemd dropin for the
+	  var.mount unit, that overrides the What and Type, and possibly
+	  the Options and After, fields.
+
+config BR2_INIT_SYSTEMD_POPULATE_TMPFILES
+	bool "trigger systemd-tmpfiles during build"
+	default y  # legacy
+	help
+	  Act on the systemd-tmpfiles.d database at build time, when
+	  assembling the root filesystems.
+
+	  This may help on a read-only filesystem.
+
+	  It probably does not play very well with the /var factory
+	  (above).
+
+config BR2_PACKAGE_SYSTEMD_DEFAULT_TARGET
+	string "The default unit systemd starts at bootup"
+	default "multi-user.target"
+	help
+	  Specify the name of the unit configuration file to be started
+	  at bootup by systemd. Should end in ".target".
+	  ex: multi-user.target
+
+	  https://www.freedesktop.org/software/systemd/man/systemd.special.html#default.target
+
+endif # BR2_INIT_SYSTEMD
+
 choice
 	prompt "/dev management" if !BR2_INIT_SYSTEMD
 	default BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS
@@ -432,7 +486,7 @@ config BR2_ENABLE_LOCALE_WHITELIST
 	default "C en_US"
 	depends on BR2_ENABLE_LOCALE_PURGE
 	help
-	  Whitespace seperated list of locales to allow on target.
+	  Whitespace separated list of locales to allow on target.
 	  Locales not listed here will be removed from the target.
 	  See 'locale -a' on your host for a list of locales available
 	  on your build host, or have a look in /usr/share/locale in
@@ -533,6 +587,16 @@ config BR2_ROOTFS_OVERLAY
 	  They are copied as-is into the rootfs, excluding files ending
 	  with ~ and .git, .svn and .hg directories.
 
+config BR2_ROOTFS_PRE_BUILD_SCRIPT
+	string "Custom scripts to run before commencing the build"
+	default ""
+	help
+	  Specify a space-separated list of scripts to be run before the
+	  build commences.
+
+	  This gives users the opportunity to do board-specific
+	  preparations before starting the build.
+
 config BR2_ROOTFS_POST_BUILD_SCRIPT
 	string "Custom scripts to run before creating filesystem images"
 	default ""