builtin/sign: add option 'keys-dir'

builtin/sign: add option 'keys-dir'

Option '--keys-dir' is used for redefinition of default directories with
public/revoked keys. If keys directory is set then default directories
are ignored and target directory is expected to contain following
structure for ed25519 signature mechanism:

dir/
  trusted.ed25519      <- file with trusted keys
  revoked.ed25519      <- file with revoked keys
  trusted.ed25519.d/   <- directory with files containing trusted keys
  revoked.ed25519.d/   <- directory with files containing revoked keys

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>