Add two optional variables for liveimage/installimage builds:
---
MBR_BOOTCODE
optional MBR bootcode which should be installed by fdisk(8)
(default: empty)
- specified MBR_BOOTCODE must exist in ${DESTDIR}/usr/mdec
- if MBR_BOOTCODE is not specified,
  MBR_DEFAULT_BOOTCODE (default: mbr) will be used
  if the target ${MACHINE} has the one in /usr/mdec

INSTALLBOOTOPTIONS
options passed to installboot(8), e.g., -o console=com0
(default: empty)
---
Proposed by Brook Milligan on current-users@:
http://mail-index.NetBSD.org/current-users/2012/04/16/msg019892.html

Also make USE_MBR work on ${MACHINE}s which don't have the default
/usr/mdec/mbr bootcode. (no such port has the liveimage target yet though)

XXX: tools fdisk(8) implicitly uses host's /usr/mdec/mbr for the default
XXX: bootcode even for non-x86 targets if host is x86

(tsutsui)