core: Reimplement lots asm code in C

core: Reimplement lots asm code in C

There is an awful lot of code currently implemented in assembly when
it could just as easily be implemented in C. Having it in C makes it
much easier to share code between the BIOS and forthcoming EFI
firmware backend. The following code fragments have been rewritten,

  - timer initialisation
  - adjust_screen()
  - check_esapes() and mem_init()
  - conio.inc
  - plaincon.inc
  - cleanup.inc
  - serirq.inc
  - font.inc
  - graphics
  - writehex

Signed-off-by: Matt Fleming <matt.fleming@intel.com>