x86: improve sysinfo handling of dummy values

x86: improve sysinfo handling of dummy values

Fall back to using board_vendor and board_name, if known dummy values
are used for sys_vendor and product_name.

Examples:
	To be filled by O.E.M.:To be filled by O.E.M.
-->	INTEL Corporation:ChiefRiver

	System manufacturer:System Product Name
-->	ASUSTeK COMPUTER INC.:P8H77-M PRO

	To Be Filled By O.E.M.:To Be Filled By O.E.M.
-->	ASRock:Q1900DC-ITX

	Gigabyte Technology Co., Ltd.:To be filled by O.E.M.
-->	Gigabyte Technology Co., Ltd.:H77M-D3H

	empty:empty
-->	TYAN Computer Corporation:TYAN Toledo i3210W/i3200R S5211

	To Be Filled By O.E.M.:To Be Filled By O.E.M.
-->	ASRock:H77 Pro4-M

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>