boot: add three new boot loader feature flags

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.