systemd: downgrade kernel headers minimum version to 3.7

The kernel headers minimum version for systemd was bumped to 3.8
(IFLA_GRE_FLOWINFO and IFLA_IPTUN_FLAGS definitions) and then to 3.10
(IFLA_VLAN_PROTOCOL definition). However, systemd since version v215
contains the following patch that defines IFLA_* constants if
needed:

  81577dc missing.h: add various network enums
  http://cgit.freedesktop.org/systemd/systemd/commit/?id=81577dc

As a consequence, this patch downgrades the kernel headers minimum
version to linux 3.7 which is required for BPF_XOR.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

system/Config.in

@@ -96,12 +96,12 @@ config BR2_INIT_SYSTEMD
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	depends on BR2_USE_MMU
 	depends on !BR2_PREFER_STATIC_LIB
-	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_7
 	select BR2_PACKAGE_SYSTEMD
 
-comment 'systemd needs an (e)glibc toolchain, headers >= 3.10'
+comment 'systemd needs an (e)glibc toolchain, headers >= 3.7'
 	depends on !(BR2_TOOLCHAIN_USES_GLIBC \
-		&& BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10)
+		&& BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_7)
 
 config BR2_INIT_NONE
 	bool "None"