8250 UART serial console

This is initial take on 8250 UART serial console to get SeaBIOS early
debugging out.

After enabling serial console debugging enabled, SeaBIOS now prints the
following at Humidor startup:

  [penberg@localhost humidor]$ go build && ./humidor --firmware ~/seabios/out/bios.bin
  Loading firmware /home/penberg/seabios/out/bios.bin... (262144 bytes)
  Changing serial settings was 0/0 now 3/0
  SeaBIOS (version rel-1.7.5-41-g3aa31d7-20140712_130259-localhost.localdomain)
  Found hypervisor signature "" at 40000000

  [snip]

  No Xen hypervisor found.
  enabling shadow ram
  IO error: port=0xcf8, direction=1, size=4
  Registers:
   rip: 00000000000ed803   rsp: 0000000000006f98 flags: 0000000000000086
   rax: 0000000080000000   rbx: 0000000000000000   rcx: 0000000080000000
   rdx: 0000000000000cf8   rsi: 0000000000000000   rdi: 0000000000000000
   rbp: 0000000000000000   r8:  0000000000000000   r9:  0000000000000000
   r10: 0000000000000000   r11: 0000000000000000   r12: 0000000000000000
   r13: 0000000000000000   r14: 0000000000000000   r15: 0000000000000000

Next step is to add proper hypervisor signature so that SeaBIOS knows
we're running on KVM.

Signed-off-by: Pekka Enberg <penberg@iki.fi>