[yocto] Manuals for ROOT LABELS INITRD and INITRD_IMAGE

[Robert Yang]

Hi Scott,

The LIVE (such as iso) and VM images (such as vmdk) can't use the same
settings for ROOT LABELS INITRD and INITRD_IMAGE, either of them may fail
to boot, so when the user uses one of them, they will get warnings like:

WARNING: core-image-minimal-1.0-r0 do_bootimg: Found potential conflicted var 
LABELS, please use LABELS_LIVE rather than LABELS"

We should use _LIVE or _VM version to instead of, would you please update
the manual for them ?

ROOT_VM: Boot loader's root options for vm images, e.g., root=/dev/sda2.
ROOT_LIVE: Boot loader's root options for live images, e.g., root=/dev/ram0.

LABELS_VM: Boot loader's actions for vm images, e.g., "boot".
LABELS_LIVE: Boot loader's actions for live images, e.g., "boot install".

INITRD_VM: Path to initial RAM disk (initrd) for vm images.
INITRD_LIVE:  Path to initial RAM disk (initrd) for live images.

# There is no warnings when use INITRD_IMAGE, I will fix it in Yocto 2.3.
INITRD_IMAGE_VM: INITRD's image name (PN) for vm images
INITRD_IMAGE_LIVE: INITRD's image name (PN) for live images

-- 
Thanks

Robert