build-setup: Fix check for MACHINE and DISTRO
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 <b20e66e0dba857f8b73ba981bbfb98d7adb86dcf@in.ibm.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts