Remove redundant declaration of mainbus_attach(), as pointed out by
Thomas Klausner.

Do not compile in the local variable "mba" in mainbus_rescan unless
NPNPBIOS > 0 || NACPI > 0 || NIPMI > 0 || NMCA > 0.  This fixes the
build on systems such as Soekris where none of those options apply.

(dyoung)