In sh, string comparison for equality is "=", not "==". Bash accepts
either operator but suggests to use "=" for posix conformance.
Our /bin/sh is currently busybox, which accepts "==" like bash, but
if you use ash or dash it fails:

   $ CONFIG_FOO=y
   $ if [ "$CONFIG_FOO" = "y" ]; then echo ok; fi
   ok
   $ if [ "$CONFIG_FOO" == "y" ]; then echo ok; fi
   dash: 3: [: y: unexpected operator

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>