kboot: Fix hostdisk_override

We were assuming that hostdisk_override was both a directory and a
file, which is not going to work very well. It's supposed to be a
single file, so recode it as such. Simplify erorr handling a little as
well and fix a return type-mismatch that doesn't matter for the
generated code (return NULL is the same as return false in this
context)

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