system: fix unmet dependencies

Don't force remounting / read/write when using a customskeleton and
systemd as init system, to fix the following unmet dependencies:

    warning: (BR2_INIT_SYSTEMD) selects BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW
    which has unmet direct dependencies (BR2_ROOTFS_SKELETON_DEFAULT)

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

system/Config.in

@@ -127,7 +127,7 @@ 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
+	select BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW if BR2_ROOTFS_SKELETON_DEFAULT
 
 comment "systemd needs a glibc toolchain, headers >= 3.10"
 	depends on !(BR2_TOOLCHAIN_USES_GLIBC \