bootctl: Always use upper case for "/EFI/BOOT" and "/EFI/BOOT/BOOT*.EFI".

If the ESP is not mounted with "iocharset=ascii", but with "iocharset=utf8"
(which is for example the default in Debian), the file system becomes case
sensitive. This means that a file created as "FooBarBaz" cannot be accessed as
"foobarbaz" since those are then considered different files.

Moreover, a file created as "FooBar" can then also not be accessed as "foobar",
and it also prevents such a file from being created, as both would use the same
8.3 short name "FOOBAR".

Even though the UEFI specification [0] does give the canonical spelling for
the files mentioned above, not all implementations completely conform to that,
so it's possible that those files would already exist, but with a different
spelling, causing subtle bugs when scanning or modifying the ESP.

While the proper fix would of course be that everybody conformed to the
standard, we can work around this problem by just referencing the files by
their 8.3 short names, i.e. using upper case.

Fixes: #3740

[0] <http://www.uefi.org/specifications>, version 2.6, section 3.5.1.1

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