x86/shim: Fix parallel build following c/s 32b1d62887d0

x86/shim: Fix parallel build following c/s 32b1d62887d0

Unfortunately, a parallel build from clean can fail in the following manner:

  xen.git$ make -j4 -C tools/firmware/xen-dir/
  make: Entering directory '/local/xen.git/tools/firmware/xen-dir'
  mkdir -p xen-root
  make: *** No rule to make target 'xen-root/xen/arch/x86/configs/pvshim_defconfig', needed by 'xen-root/xen/.config'.  Stop.
  make: *** Waiting for unfinished jobs....

The rule for pvshim_defconfig needs to depend on the linkfarm, rather than
$(D)/xen/.config specifically.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>