utils/genrandconfig: add mxs-bootlets custom handling

Add custom cases to make sure that a random configuration with an empty
git or tarball location for mxs-bootlets doesn't fail. It reverts to
BR2_TARGET_MXS_BOOTLETS_FREESCALE in that case.

Fixes:
 - http://autobuild.buildroot.org/results/dcbeb73d152c79c18b1fa3bef3b4fa07635d7b36
 - http://autobuild.buildroot.org/results/f61cf3fc58db9ded5ec42ebf4a9847584700698d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>