selftests/ima: kexec_load syscall test

The kernel CONFIG_KEXEC_VERIFY_SIG option is limited to verifying a
kernel image's signature, when loaded via the kexec_file_load syscall.
There is no method for verifying a kernel image's signature loaded
via the kexec_load syscall.

This test verifies loading the kernel image via the kexec_load syscall
fails when the kernel CONFIG_KEXEC_VERIFY_SIG option is enabled on
systems with secureboot enabled[1].

[1] Detecting secureboot enabled is architecture specific.

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>