RISC-V: Cosmetic menuconfig changes

* Move the built-in cmdline configuration on a new menu entry "Boot
  options", it doesn't make much sense to be part of the debuging menu.

* Rename "Kernel Type" menu to "Kernel features" to be more consistent with
  what other architectures are using, plus "type" is a bit misleading here.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>