selftest/kexec: fix "ignored null byte in input" warning

Instead of assigning the string to a variable, which might contain a
null character, redirect the output and grep for the string directly.
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

tools/testing/selftests/kexec/test_kexec_file_load.sh

@@ -97,10 +97,11 @@ check_for_imasig()
 check_for_modsig()
 {
 	local module_sig_string="~Module signature appended~"
-	local sig="$(tail --bytes $((${#module_sig_string} + 1)) $KERNEL_IMAGE)"
 	local ret=0
 
-	if [ "$sig" == "$module_sig_string" ]; then
+	tail --bytes $((${#module_sig_string} + 1)) $KERNEL_IMAGE | \
+		grep -q "$module_sig_string"
+	if [ $? -eq 0 ]; then
 		ret=1
 		log_info "kexec kernel image modsig signed"
 	else