system.sh.sample: fix bashism

script system.sh.sample does not appear to be a /bin/sh script
possible bashism in system.sh line 7 (should be 'b = a'):
if [ "x${ARCH}" == "xarmv7l" ] ; then

Pointed out by: armde@elektromaniak.wz.cz

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>