xen/build: use the correct kconfig makefile

xen/build: use the correct kconfig makefile

This resolves the following observed error during config merge:

  /bin/sh /path/to/xen/xen/../xen/tools/kconfig/merge_config.sh -m .config /path/to/xen/xen/../xen/arch/arm/configs/custom.config
  Using .config as base
  Merging /path/to/xen/xen/../xen/arch/arm/configs/custom.config
  #
  # merged configuration written to .config (needs make)
  #
  make -f /path/to/xen/xen/../xen/Makefile olddefconfig
  make[2]: Entering directory '/path/to/xen/xen'
  make[2]: *** No rule to make target 'olddefconfig'.  Stop.
  make[2]: Leaving directory '/path/to/xen/xen'
  tools/kconfig/Makefile:95: recipe for target 'custom.config' failed

The build was invoked by first doing a defconfig (which succeeded):

  $ make -C xen XEN_TARGET_ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- defconfig

Followed by the config fragment merge command (which failed before this patch)

  $ cat > xen/arch/arm/configs/custom.config <<EOF
  CONFIG_DEBUG=y
  CONFIG_EARLY_PRINTK_ZYNQMP=y
  EOF
  $ make -C xen XEN_TARGET_ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- custom.config

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>