more robust system serial number detection

...for BIOSes which don't report system-serial-number correctly. My reasoning behind using baseboard-serial-number rather than system-serial-number is that all the non-"Enterprise" kit that i've tried either doesn't report a system-serial-number or reports a silly/useless string (like Supermicro does):

Intel NUC6i5SYB:

baseboard-product-name: NUC6i5SYB
baseboard-serial-number: GESY633000YB
system-serial-number: <blank string>

My Thinkpad X1C does report most things correctly, however:

system-serial-number: PF0XXXX
baseboard-serial-number: L3HFXXXXXXX
baseboard-product-name: 20HRCTO1WW
baseboard-manufacturer: LENOVO

Yet another Consumer motherboard:

system-serial-number: System Serial Number
baseboard-serial-number: 180221257600801
baseboard-product-name: ROG STRIX B360-I GAMING
baseboard-manufacturer ASUSTeK COMPUTER INC.

Unfortunately the problem is absolutely pathological elsewhere:

No-name Chinese i7:

system-serial-number To be filled by O.E.M.
baseboard-serial-number To be filled by O.E.M.
baseboard-product-name To be filled by O.E.M.
baseboard-manufacturer: To be filled by O.E.M`

Gigabyte A320M-H-CF

system-serial-number: Default string
baseboard-serial-number: Default string
baseboard-product-name: A320M-H-CF
baseboard-manufacturer: Gigabyte Technology Co., Ltd.