kernel: get rid of physical page bank array and use conf.mem[] instead

kernel: get rid of physical page bank array and use conf.mem[] instead

We can take advantage of the fact that xinit() allocates
kernel memory from conf.mem[] banks always at the beginning
of a bank, so the separate palloc.mem[] array can be eleminated
as we can calculate the amount of non-kernel memory like:

upages = cm->npage - (PGROUND(cm->klimit - cm->kbase)/BY2PG)

for the number of reserved kernel pages,
we provide the new function: ulong nkpages(Confmem*)

This eleminates the error case of running out of slots in
the array and avoids wasting memory in ports that have simple
memory configurations (compared to pc/pc64).