qemu-check.exp needs the environment variable $XEN_BOOT to be set,
or the following error is displayed:
 $ make check
 ...
 # no such variable
     (read trace on "::env(XEN_BOOT)")
     invoked from within
 "if {$::env(XEN_BOOT) == "y"} {
         info " (Xen Dom0)"
 }"
     (file "/<...>/qemu-check.exp" line 119)

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Acked-by: Ruchika Gupta <ruchika.gupta@linaro.org>