Revert "generic-board: allow showing custom board info"

Drop CONFIG_CUSTOM_BOARDINFO as it is not Kconfig compliant and anyway
not really used anywhere plus the upcoming weak show_board_info()
approach seems much superior.

This reverts commit a9ad18c9.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>