rework firmware menu, we need to add some more options for images/filesystem

Config.in

@@ -25,7 +25,12 @@ source "target/config/Config.in.subsystem"
 source "target/config/Config.in.qemuopts"
 source "target/config/Config.in.cpu"
 source "target/config/Config.in.tasks"
+
+menu "Firmware configuration"
+	visible if ADK_TARGET_OS_LINUX && !ADK_APPLIANCE_TOOLCHAIN && !ADK_TARGET_CHOOSE_ARCH
 source "target/config/Config.in.rootfs"
+endmenu
+
 source "target/config/Config.in"
 
 menu "Package selection"

target/config/Config.in.rootfs

@@ -2,7 +2,7 @@
 # material, please see the LICENCE file in the top-level directory.
 
 choice
-prompt "Firmware type" if !ADK_TARGET_CHOOSE_ARCH
+prompt "Filesystem / Image type" if !ADK_TARGET_CHOOSE_ARCH
 depends on !ADK_APPLIANCE_TOOLCHAIN && !ADK_TARGET_OS_BAREMETAL
 default ADK_TARGET_ROOTFS_INITRAMFSPIGGYBACK if ADK_TARGET_QEMU || ADK_TARGET_SIM
 default ADK_TARGET_ROOTFS_ARCHIVE
@@ -170,8 +170,12 @@ config ADK_TARGET_ROOTFS_GENIMAGE
 endchoice
 
 config ADK_TARGET_ROOTDEV
-	string
+	string "Root device name"
+	depends on ADK_TARGET_ROOTFS_GENIMAGE
 	default "sda"
+	help
+	  If you a embedded device with multiple block devices and want to
+	  set the root device name.
 
 config ADK_TARGET_DUAL_BOOT
 	bool "Activate dual-boot partition layout"