ARM: shmobile: r7s72100: Remove legacy board support

There's no legacy board anymore, genmai now boots with multiplatform
support only. Remove the leftovers.

Makefile.boot portion pointed out by Paul Bolle.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Wolfram Sang <wsa@sang-engineering.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
[horms+renesas@verge.net.au: squashed in patch containing
 Makefile.boot change]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>