baremetal: allow arbitrary exit status with the magic string

test-baremetal: fix missing setting x0 return value

Examples were just returning on ret without setting x0, which led to
failures... those were not noticed because of how broken the testing system
was ;-)

rootfs_overlay/lkmc/test_fail.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 # https://github.com/cirosantilli/linux-kernel-module-cheat#magic-failure-string
-echo lkmc_test_fail
+echo lkmc_exit_status_1