User-mode does not need the system tools. Do not build them by
default if the user specifies --disable-system.

This disables building the following binaries on a user-only build:

- elf2dmp
- qemu-edid
- qemu-ga
- qemu-img
- qemu-io
- qemu-nbd
- ivshmem-client
- ivshmem-server

The qemu-user binaries are not affected by this change.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200217133327.25144-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>