Adjust uki-build.go checks to allow any DER files

[kreeuwijk]

We are switching to generating the DER files for Secure Boot with sig-list-to-certs. This creates numbered DER files from each ESL: PK-0.der KEK-0.der KEK-1.der db-0.der db-1.der db-2.der ...

However build-uki.go has hardcoded checks for PK.der, KEK.deranddb.der`. As these files are inconsequential for automatic enrollment, any available DER files should just be copied to the ISO.

Please adjust the checks so it looks for any DER files instead of hardcoded ones.

[bencorrado]

It also is currently picky about the case of the filename, which other systems do not seem to care about in my experience.