generic-board: show model name in board_init_f() too

The common/board_r.c has show_model_r() to display the model name
if the DTB has a "model" property.  It sounds useful to have a similar
function in common/board_f.c too because most of the boards show
their board name before relocation.

Instead of implementing the same function in both common/board_f.c
and common/board_r.c, let's split it up into common/show_board_info.c.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>