Revert "generic-board: make show_board_info a weak function"

We already have:

- a sysinfo driver-interface which can obtain the model
- a weak function called checkboard() for showing board info

The current implementation has a weak function on top of a weak
function. Now that all boards have been updated to use checkboard()
instead, drop the __weak on show_board_info()

This reverts commit f7637cc0.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>