sparc: consolidate CONFIG_{LEON, LEON2, LEON3} definition

CONFIG_LEON is already defined in
arch/sparc/cpu/{leon2,leon3}/config.mk.
Remove the redundant definition in board header files.

All leon3 boards define CONFIG_LEON3 in board header files.
Move the definition to arch/sparc/cpu/leon3/config.mk.

CONFIG_LEON2 can be move to arch/sparc/cpu/leon2/config.mk
as well.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>