buildroot: move BR2_ROOTFS_{PREFIX,SUFFIX} to target menu

As suggested on the list by Thiago - Makes more sense to have this together
with the stuff using it.

Config.in

@@ -116,18 +116,6 @@ config BR2_TOPDIR_SUFFIX
 	  build_ARCH -> build_ARCH_[SUFFIX]
 	  toolchain_build_ARCH -> toolchain_build_ARCH_[SUFFIX]
 
-config BR2_ROOTFS_PREFIX
-	string "Custom root fs prefix"
-	default "rootfs"
-	help
-	  Add a custom string to the beginning of the root file system name.
-
-config BR2_ROOTFS_SUFFIX
-	string "Custom root fs suffix"
-	default ""
-	help
-	  Add a custom string to the end of the root file system name.
-
 config BR2_GNU_BUILD_SUFFIX
 	string "GNU build hostname suffix"
 	default "pc-linux-gnu"

target/Config.in

@@ -1,5 +1,17 @@
 menu "Target filesystem options"
 
+config BR2_ROOTFS_PREFIX
+	string "Custom root fs prefix"
+	default "rootfs"
+	help
+	  Add a custom string to the beginning of the root file system name.
+
+config BR2_ROOTFS_SUFFIX
+	string "Custom root fs suffix"
+	default ""
+	help
+	  Add a custom string to the end of the root file system name.
+
 comment "filesystem for target device"
 
 source "target/cramfs/Config.in"