build-setup: Fix check for MACHINE and DISTRO

The script checks if variables MACHINE and DISTRO
both have non-empty values. The check used a '-n'
test. This commit fixes it by using '-z'

Change-Id: Ic4230760af3b8c0f15e013e3c5d24154619e4bdf
Signed-off-by: Santosh Puranik <santosh.puranik@in.ibm.com>
diff --git a/build-setup.sh b/build-setup.sh
index ea35b29..1a2ee1c 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -343,7 +343,7 @@
 # Source our build env
 ${BITBAKE_CMD}
 
-if [[ -n "${MACHINE}" || -n "${DISTRO}" ]]; then
+if [[ -z "${MACHINE}" || -z "${DISTRO}" ]]; then
   echo "MACHINE or DISTRO is not configured for ${target}"
   exit 1
 fi