imgtool: Remove P224 curve references

Imgtool does not provide support for P224
curve, just a placeholder function that
says it's not currently implemented.
This has now been removed as P224 support
had been removed from the boot code too.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: I477d8e273085f38f35eaf9a591584f3e937d748d