[digest] Add "--set" option to store digest value in a setting

[digest] Add "--set" option to store digest value in a setting

Allow the result of a digest calculation to be stored in a named
setting.  This allows for digest verification in scripts using e.g.:

  set expected:hexraw cb05def203386f2b33685d177d9f04e3e3d70dd4
  sha1sum --set actual 1mb
  iseq ${expected} ${actual} || goto checksum_bad

Note that digest verification alone cannot be used to set the trusted
execution status of an image.  The only way to mark an image as
trusted is to use the "imgverify" command.

Signed-off-by: Michael Brown <mcb30@ipxe.org>