"boot.config" (relative to the root directory on the 'a' partition
on the first BSD slice) if it exists. If it doesn't exist, then
the only visible changes should be that the kernel name isn't reset
to "/kernel" after looking it up fails and that the default name
is now "kernel".
The new function readfile() can be used for other things:
- reading help messages.
- reading splash screens.
- reading userconfig info.
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