The line disabling CONFIG_JFFS2_FS was removed by mistake by commit
a6d968c8 ("qemu: generic driver"). Restore it to fix the Travis
tests.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>