imgtool: Fix getpub fails for ed25519 key

Fix getpub command fails to emit public pair of ed25519 key with
encoding.

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I3d255f576b2d8a8d0cbc8e31ff378717acb060c7
(cherry picked from commit ef598b13)