HP EliteBook EFI boot failure

[Greg Rivers]

I'm unable to boot the memstick EFI images on a HP EliteBook 850 G1 (SKU 
F2Q24UT#ABA). The BIOS is up to date and set to the default "UEFI Native 
(Without CSM)" setting.

I tried the most recent 10.1-STABLE and 11.0-CURRENT snapshots with the 
following results (hand transcribed from the screen):

FreeBSD-10.1-STABLE-amd64-20150309-r279796-uefi-memstick.img.xz:
--------------------------------------------------------------------------------
>> FreeBSD EFI boot block
    Loader path: /boot/loader.efi
Consoles: EFI console
Image base: 0x9fb5e000
EFI version 2.10
EFI Firmware: HPQ (rev 4096.01)

FreeBSD/amd64 EFI loader, Revision 1.1
(root at releng1.nyi.freebsd.org, Mon Mar  9 16:10:08 UTC 2015)
Loading /boot/defaults/loader.conf
/boot/kernel/kernel text=0xf9d8c8 data=0x125ce8 syms=[0x8+0x141318+0x8+0x15be03]

Hit [Enter] to boot immediately, or any other key for command prompt.
Booting [/boot/kernel/kernel]...
Start @ 0xffffffff802dbc90 ...
--------------------------------------------------------------------------------

FreeBSD-11.0-CURRENT-amd64-20150309-r279813-memstick.img.xz:
--------------------------------------------------------------------------------
>> FreeBSD EFI boot block
    Loader path: /boot/loader.efi
Consoles: EFI console
Image base: 0x9fa06000
EFI version 2.10
EFI Firmware: HPQ (rev 4096.01)

FreeBSD/amd64 EFI loader, Revision 1.1
(root at releng2.nyi.freebsd.org, Mon Mar  9 16:24:01 UTC 2015)
Loading /boot/defaults/loader.conf
/boot/kernel/kernel text=0x103a3c0 data=0x3faef0 syms=[0x8+0x148f38+0x8+0x164ae7]

Hit [Enter] to boot immediately, or any other key for command prompt.
Booting [/boot/kernel/kernel]...
Start @ 0xffffffff802e4000 ...
EFI framebuffer information:
addr, size	0xc0000000, 0x7e9000
dimensions	1920 x 1080
stride		1920
masks		0x0ff0000, 0x000000ff, 0xff000000
--------------------------------------------------------------------------------

In both cases the system simply hangs.  How can I help debug this?

-- 
Greg Rivers