add support to efifb for drawing a console on a coreboot framebuffer

add support to efifb for drawing a console on a coreboot framebuffer
if coreboot's memory table is found, it has a framebuffer entry, and
there is no previously attached efi, vga, or serial console.

useful on chromebooks that have no legacy vga device to get an early
console before inteldrm(4) attaches or, for newer chipsets, a full
console and X with wsfb(4).

ok kettenis