How to manually boot OS X with Grub2 by SMX

Instructions by SMX
How to boot OS X from usb without usb booting support in BIOS with grub2
We go to command line
press 'c' to bring up console if you are in the menu

We insert usb modules
insmod usb
insmod ohci
insmod uhci
insmod ehci
insmod usbms   --> this for usb mass storage

We insert partition and filesystem modules
insmod part_mbr
insmod part_gpt
insmod hfsplus

We find devices
usb         --> check output for your device
ls          --> check output for (usb) devices

Identify device and do
ls -l (usb1b,gpt2)    --> example, replace usb1b,gpt2 with yours (same in the rest of tutorial)

Write down UUID
Export uuid to a global variable

set uuid=YOURUUID --> replace YOURUUID with the one you wrote down
export uuid

Now change root to your device
set root=(usb1b,gpt2)

Check files are there with
ls /

Set xnu uuid with
xnu_uuid $uuid uuid

Load mach kernel and parameters with
xnu_kernel /mach_kernel boot-uuid=${uuid} rd=*uuid -v      --> insert any other parameter you need here

If you are booting a 64bit kernel you instead need
xnu_kernel64 /mach_kernel boot-uuid=${uuid} rd=*uuid -v    --> insert any other parameter you need here

Load Extensions:
Option A: use caches:
xnu_mkext /System/Library/Extensions.mkext
xnu_mkext /Extra/Extensions.mkext

Option B: use folders:
xnu_kextdir /System/Library/Extensions
xnu_kextdir /Extra/Extensions

Optional: load Custom DSDT
acpi -e /Extra/DSDT.aml

Boot the system with
boot