by looking it up in the x86_alldisks table (instead of trying to match it
to 'wd*' manually).
In order to do this, move the cpu_rootconf function from x86 common code
to amd64 and i386 specific one. This way, i386 can do an extra step (call
the appropriate Multiboot code) in the appropriate place (after
x86_matchbiosdisks and before findroot()).
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