target: target_get_name() --> target_type_name()

There are two names that may matter on a per-target basis.
One is a per-instance name (for example, "at91sam7s.cpu").
The other is the name of its type (for example, "arm7tdmi"),
which is shared among multiple targets.

Currently target_get_name() returns the type name, which is
misleading and is rarely appropriate for target diagnostics.
Rename that as target_type_name().

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>