system: systemd only really supports a R/W rootfs

Supporting a R/O rootfs needs a bit more love to be fully operational.
This will come in later patches...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

system/Config.in

@@ -94,6 +94,7 @@ config BR2_INIT_SYSTEMD
 	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
 	select BR2_ROOTFS_MERGED_USR
 	select BR2_PACKAGE_SYSTEMD
+	select BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW
 
 comment "systemd needs a glibc toolchain, headers >= 3.10"
 	depends on !(BR2_TOOLCHAIN_USES_GLIBC \