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.

src/cryptsetup/cryptsetup.c

@@ -828,9 +828,9 @@ static int measure_volume_key(
                 return 0;
         }
 
-        r = efi_stub_measured();
+        r = efi_stub_measured(LOG_WARNING);
         if (r < 0)
-                return log_warning_errno(r, "Failed to detect if we are running on a kernel image with TPM measurement enabled: %m");
+                return r;
         if (r == 0) {
                 log_debug("Kernel stub did not measure kernel image into the expected PCR, skipping userspace measurement, too.");
                 return 0;