utils/genrandconfig: add ti-k3-r5-loader handling

Fix the following build failure raised since the addition of the package
in commit 0189bcb4:

make[2]: *** No rule to make target '_defconfig'.  Stop.

Fixes:
 - http://autobuild.buildroot.org/results/d7449b2b2f2349af672bfeee832b89a223a7d9cc



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>