imgtool: Improve ECDSA key generation

This patch improves the existing ECDSA key generation feature
in the imgtool by:
 - Fix a bug in the 'minimal' representation of PKCS#8 keys where
   the resulting ASN.1 DER encoding is not compliant
 - Add the option to export ECDSA private keys in SEC1 format by
   providing a command line option -f or --format that can be
   'openssl' (for SEC1 format) or 'pkcs8'. This format ends up in
   key encodings which are generally smaller than PKCS#8.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>