system: Warn if systemd is used with kernel < 4.15

To quote the README:

> Kernel versions below 4.15 have significant gaps in functionality and
> are not recommended for use with this version of systemd. Taint flag
> 'old-kernel' will be set. Systemd will most likely still function, but
> upstream support and testing are limited.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Reviewed-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

system/Config.in

@@ -140,6 +140,10 @@ comment "systemd needs a glibc toolchain w/ SSP, headers >= 3.15, host and targe
 		!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