clear bss

I had problems with uncleared bss. E.g. the static variable pt_inuse was not set to zero. 
I think I have found the reason for it.

The values were wrong for:
__k_unpaged__edata
__k_unpaged__end

I think the unpaged bss  _kern_unpaged_edata ... _kern_unpaged_edata has to be cleared too.

Therefore the paged bss:       _edata ... _end
and the unpaged bss: _kern_unpaged_edata ... _kern_unpaged_edata
are both cleared now in the function pre_init.

The values can be verified by:

${CROSS_TOOLS}/arm-elf32-minix-objdump -xD ${OBJ}/kernel/kernel