qemu: "make check": make number of cores configurable

The QEMU_SMP environment variable or makefile macro may be set to
control the number of cores used during "make check".

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/qemu-check.exp b/qemu-check.exp
index ba5905b..c598e30 100644
--- a/qemu-check.exp
+++ b/qemu-check.exp
@@ -42,7 +42,7 @@
 open "serial1.log" "w+"
 spawn -open [open "|tail -f serial1.log"]
 set teecore $spawn_id
-spawn $::env(QEMU) -nographic -monitor none -machine virt -machine secure=on -cpu cortex-a15 -smp 4 -d unimp -semihosting-config enable,target=native -m 1057 -serial stdio -serial file:serial1.log -bios $bios
+spawn $::env(QEMU) -nographic -monitor none -machine virt -machine secure=on -cpu cortex-a15 -smp $::env(QEMU_SMP) -d unimp -semihosting-config enable,target=native -m 1057 -serial stdio -serial file:serial1.log -bios $bios
 expect {
 	"Kernel panic" {
 		info "!!! Kernel panic\n"