boot: Restore and extend the usage of 0x22 TLV (ECDSA256)

The IMAGE_TLV_ECDSA256 TLV has been put out of use by
commit 63d2346d.
This commit reverts this part of that patch and at the
same time it extends the usage of this TLV to cover all types
of curves (replacing the newly introduced 0x25 TLV type)
while retaining its value (0x22) for backward compatibility.
Rename IMAGE_TLV_ECDSA256 to IMAGE_TLV_ECDSA_SIG.

Change-Id: I904f292db775c38f26a5e9a87c5f414165efc173
Signed-off-by: David Vincze <david.vincze@arm.com>