kconfigs/qemu.conf: disable CONFIG_JFFS2_FS

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>

kconfigs/qemu.conf

 CONFIG_TEE=y
 CONFIG_OPTEE=y
-### This prevents random failures of "make check" in Travis CI
+### Enabling PREEMPT and disabling JFFS2_FS prevents random failures of
+### "make check" in Travis CI
 CONFIG_PREEMPT=y
+CONFIG_JFFS2_FS=n