qemu: disable kernel CONFIG_JFFS2_FS

We have observed random failures of "make check" when run in the
Travis environment (OP-TEE regression testing). Every time it fails,
the log file has the following errors [1]:

  Starting QEMU...send: spawn id exp7 not open
      while executing
  "send -- "\r""
      (file "qemu-check.exp" line 54)
  == /home/travis/optee_repo/build/serial0.log:
  spawn ../qemu/arm-softmmu/qemu-system-arm -nographic [...]
  Using QEMU provided DTB at 0x40000000
  [...]
  squashfs: version 4.0 (2009/01/31) Phillip Lougher
  jffs2: version 2.2. (NAND) send: spawn id exp7 not open
      while executing
  "send -- "\r""
      (file "qemu-check.exp" line 54)
  == end of /home/travis/optee_repo/build/serial0.log

It seems that QEMU exits unexpectedly as the kernel is initializing
the JFFS2 filesystem driver. Since this is something we don't need, I
thought I'd just disable this driver and see if it makes things more
stable.
Well, indeed I could not reproduce the issue with this patch applied.

[1] https://travis-ci.org/OP-TEE/optee_os/builds/90051975#L4577



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