doc: fix warnings in apps_kernel_conf.rst

- :option:`XYZ` can only be used if there is an .. option: XYZ
  definititon somewhere; replaced with a more generic monospace
  ``XYZ``.

- Fixed broken link to 'Creating and Configuring an MDEF File for a
  Microkernel Application'.

Change-Id: I2b1e862415035500c00dcce0a33d14bac4126044
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>