loader.efi: Parse SPCR table entry in ACPI tables

[imp]

Summary

If there's a SPCR, then use it to create and pass the right values to
the uart.

Ideally, I'd just pass this all in hw.uart.console, but old kernels
completely ignore the value if there's any extra entries. So I have to
pass newly invented values to replace kernel values in a new kenv
hw.uart.console.extra. so we pass xo=-1 in to calcuate the xo from the
baud rate, and pv and pd for PCI Vendor ID and PCI Device ID of the
serial console to use. Old kernels will ignore this and completely get
this data from parsing SPCR itself. New kernels will dig it our of here
w/o needing to parse it.

We don't support Rev3 or Rev4 SPCR yet. It's too new to be in real
hardware yet.

Sponsored by: Netflix

[andrew]

stand/efi/loader/main.c

797–801

Are these ACPI_DBG2_* values?

[imp]

stand/efi/loader/main.c

797–801

Both DBG2 and SPCR use the same tables, from the InterfaceType spec:
"For Revision 2 or higher: See the Serial Port Subtypes in Table 3 of the DBG2 Specification."
which is where I snagged these values from. I don't know which are active in the wild, but I have observed '1' and '3' on systems I have here.

[imp]

fix checkstyle9.pl issues