boot: add three new boot loader feature flags

Report whether the devicetree + sort-key boot loader spec type #1
fields are supported, and whether the "@saved" pseudo-entry is
supported.

Strictly speaking, thes features have been added in versions that are
already released (250+), so by adding this those version even though
they support the features will be considered not supporting them, but
that should be OK (the opposite would be a problem though, i.e. if we'd
assume a boot loader had a feature it actually does not).

These three features are features relevant to userspace, as it allows
userspace to tweak/genereate BLS entries or set EFI vars correctly.
Other features (i.e. that have no impliciations to userspace) are not
reported.

src/boot/bootctl.c

@@ -1723,6 +1723,9 @@ static int verb_status(int argc, char *argv[], void *userdata) {
                         { EFI_LOADER_FEATURE_XBOOTLDR,                "Support for XBOOTLDR partition"        },
                         { EFI_LOADER_FEATURE_RANDOM_SEED,             "Support for passing random seed to OS" },
                         { EFI_LOADER_FEATURE_LOAD_DRIVER,             "Load drop-in drivers"                  },
+                        { EFI_LOADER_FEATURE_SORT_KEY,                "Support Type #1 sort-key field"        },
+                        { EFI_LOADER_FEATURE_SAVED_ENTRY,             "Support @saved pseudo-entry"           },
+                        { EFI_LOADER_FEATURE_DEVICETREE,              "Support Type #1 devicetree field"      },
                 };
                 static const struct {
                         uint64_t flag;

src/boot/efi/boot.c

@@ -2443,6 +2443,9 @@ static void export_variables(
                 EFI_LOADER_FEATURE_XBOOTLDR |
                 EFI_LOADER_FEATURE_RANDOM_SEED |
                 EFI_LOADER_FEATURE_LOAD_DRIVER |
+                EFI_LOADER_FEATURE_SORT_KEY |
+                EFI_LOADER_FEATURE_SAVED_ENTRY |
+                EFI_LOADER_FEATURE_DEVICETREE |
                 0;
 
         _cleanup_free_ char16_t *infostr = NULL, *typestr = NULL;

src/fundamental/efivars-fundamental.h

@@ -13,6 +13,9 @@
 #define EFI_LOADER_FEATURE_XBOOTLDR                (UINT64_C(1) << 5)
 #define EFI_LOADER_FEATURE_RANDOM_SEED             (UINT64_C(1) << 6)
 #define EFI_LOADER_FEATURE_LOAD_DRIVER             (UINT64_C(1) << 7)
+#define EFI_LOADER_FEATURE_SORT_KEY                (UINT64_C(1) << 8)
+#define EFI_LOADER_FEATURE_SAVED_ENTRY             (UINT64_C(1) << 9)
+#define EFI_LOADER_FEATURE_DEVICETREE              (UINT64_C(1) << 10)
 
 /* Features of the stub, i.e. systemd-stub */
 #define EFI_STUB_FEATURE_REPORT_BOOT_PARTITION     (UINT64_C(1) << 0)