package/qemu: allow building only the tools

Commit 44be514b21fd (package/qemu: refactor target emulator selection)
mistakenly forced to always build at least user or system emulation but
it should be possible to choose only the tools (e.g. for a guest VM
image).

Still, we don't want to configure qemu with neither system nor user
emulation, nor the tools enabled; at least one lust be enabled.

So, change the selection to force the tools to be enabled if neither the
system nor the user emulations are enabled.

Reinstate the the machine selection guard behind a dependency on whether
at lesat system or user emulation is enabled.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
[yann.morin.1998@free.fr:
  - ensure at least one of system, user, or tools
  - guard the machine selection
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/qemu/Config.in

@@ -27,7 +27,7 @@ menuconfig BR2_PACKAGE_QEMU
 	select BR2_PACKAGE_LIBGLIB2
 	select BR2_PACKAGE_PIXMAN
 	select BR2_PACKAGE_ZLIB
-	select BR2_PACKAGE_QEMU_SYSTEM if !BR2_PACKAGE_QEMU_LINUX_USER
+	select BR2_PACKAGE_QEMU_TOOLS if !BR2_PACKAGE_QEMU_SYSTEM && !BR2_PACKAGE_QEMU_LINUX_USER
 	help
 	  QEMU is a generic and open source machine emulator and
 	  virtualizer.
@@ -114,6 +114,7 @@ comment "Linux user-land emulation needs a glibc or uClibc toolchain"
 
 config BR2_PACKAGE_QEMU_CHOOSE_TARGETS
 	bool "Select individual emulator targets"
+	depends on BR2_PACKAGE_QEMU_SYSTEM || BR2_PACKAGE_QEMU_LINUX_USER
 	help
 	  By default, all targets (system and/or user, subject to the
 	  corresponding options, above) are built. If you only need a