In preparation for the riscv toolchain, which uses suffix .i for object files,
some pc kernel firmware files named with suffix .i need to be renamed, so that
they won't be deleted by 'mk clean' (which deletes all object files). These
firmware files contain C #include source, so it makes sense to name them with
suffix .h instead.

Before or after applying this patch, the following command also needs to be run:

  for (file in etherm10g2k etherm10g4k sd53c8xx uartaxp) mv $file.i $file.h