r336921 broke booting on MBP 2017, EFIRT related

[Yuri Pankov]

Hi,

I've noticed that all recent snapshots (ALPHA3, ALPHA2, ALPHA1, 
20180802) fail to boot on MBP 2017:

kbd0 at kbdmux0
netmap: loaded module
nexus0

Fatal trap 12: page fault while in kernel mode
cpuid = 2: apic id = 02
fault virtual address  = 0x74c64a50
fault code             = supervisor read data, page not present
instruction pointer    = 0x20: 0x7abece31
stack pointer          = 0x28: 0xffffffff82b2f7c0
frame pointer          = 0x28: 0xffffffff82b2f810
code segment           = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags       = interrupt enabled, resume, IOPL = 0
current process        = 0 (swapper)
[ thread pid 0 tid 100000 ]
Stopped at      0x7abece31:    calll   *0x18(%rax)
db>

Sadly, there's no support for internal keyboard yet (it's connected via 
SPI), and external USB one stops working.

A (not so quick) bisect is pointing at r336921, which enabled EFIRT.

Some questions here:
- is this something that can/should be fixed?
- can we print some "enabling EFIRT" message to the console to make
   guesses about the problem source a bit easier?