add generic kremap for kernel remapping

* add generic utility to allocate pagetable from static pool for initial
  mmu setup
* fix incorrect pte advancement in set_ptes_contig
* always dump the configured headers when performing `make config`
* add custom qemu trace event for logging pagefaults in x86
* refactor the generic parts of bstage.h to asm-generic/boot_stage.h
* make the rootfs accessible if `make root` is invoked directly from
  sudo