hw/core: Only build guest-loader if libfdt is available

hw/core: Only build guest-loader if libfdt is available

Add a Kconfig entry for guest-loader so we can optionally deselect
it (default is built in), and add a Meson dependency on libfdt.

This fixes when building with --disable-fdt:

  /usr/bin/ld: libcommon.fa.p/hw_core_guest-loader.c.o: in function `loader_insert_platform_data':
  hw/core/guest-loader.c:56: undefined reference to `qemu_fdt_add_subnode'
  /usr/bin/ld: hw/core/guest-loader.c:57: undefined reference to `qemu_fdt_setprop'
  /usr/bin/ld: hw/core/guest-loader.c:61: undefined reference to `qemu_fdt_setprop_string_array'
  /usr/bin/ld: hw/core/guest-loader.c:68: undefined reference to `qemu_fdt_setprop_string'
  /usr/bin/ld: hw/core/guest-loader.c:74: undefined reference to `qemu_fdt_setprop_string_array'
  collect2: error: ld returned 1 exit status

Fixes: a33ff6d2c6b ("hw/core: implement a guest-loader to support static hypervisor guests")
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210315170439.2868903-1-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>