shared/efi-loader: fix compilation with !ENABLE_EFI, improve messages

shared/efi-loader: fix compilation with !ENABLE_EFI, improve messages

When compiled without ENABLE_EFI, efi_stub_measured() was not defined, so
compilation would fail. But it's not enough to add a stub that returns
-EOPNOTSUPP. We call this function in various places and usually print the error
at warning or error level, so we'd print a confusing message. We also can't add
a stub that always returns 0, because then we'd print a message like "Kernel
stub did not measure", which would be confusing too. Adding special handling for
-EOPNOTSUPP in every caller is also unattractive. So instead efi_stub_measured()
is reworked to log the warning or error internally, and such logging is removed
from the callers, and a stub is added that logs a custom message.