01fips: Properly fix creating path to .hmac of kernel based on BOOT_IMAGE

01fips: Properly fix creating path to .hmac of kernel based on BOOT_IMAGE

8f5c5 broke the case where BOOT_IMAGE is not set at all.
This code should handle following:
1) BOOT_IMAGE not set
2) BOOT_IMAGE set to something unrelated (s390)
3) BOOT_IMAGE=vmlinuz-4.14.7-300.fc27.x86_64
4) BOOT_IMAGE=/vmlinuz-4.14.7-300.fc27.x86_64
5) BOOT_IMAGE=/boot/vmlinuz-4.14.7-300.fc27.x86_64
6) BOOT_IMAGE=subdir/vmlinuz-4.14.7-300.fc27.x86_64
7) BOOT_IMAGE=/subdir/vmlinuz-4.14.7-300.fc27.x86_64
8) BOOT_IMAGE=/boot/subdir/vmlinuz-4.14.7-300.fc27.x86_64

https://bugzilla.redhat.com/show_bug.cgi?id=1415032