utils/genrandconfig: fix flake8

Commit af494d92d3d0 (utils/genrandconfig: disable libopenssl without
atomics) intorduced a code-style issue that flake8 does not like:

    $ make check-flake8
    utils/genrandconfig:253:8: E713 test for membership should be 'not in'
    1     E713 test for membership should be 'not in'

Fixes: af494d92d3d0
    https://gitlab.com/buildroot.org/buildroot/-/jobs/3045260108

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

utils/genrandconfig

@@ -250,7 +250,7 @@ def fixup_config(sysinfo, configfile):
     # libopenssl needs atomic, but propagating this dependency in
     # Buildroot is really too much work, so we handle this here.
     if 'BR2_PACKAGE_LIBOPENSSL=y\n' in configlines and \
-       not 'BR2_TOOLCHAIN_HAS_ATOMIC=y\n' in configlines:
+       'BR2_TOOLCHAIN_HAS_ATOMIC=y\n' not in configlines:
         return False
     if 'BR2_PACKAGE_SUNXI_BOARDS=y\n' in configlines:
         configlines.remove('BR2_PACKAGE_SUNXI_BOARDS_FEX_FILE=""\n')