Fetch internal disk when using removable disk

It always uses the current boot kernel now. In kernel_handler, add an argument
"internal_disk" to decide which disk to retrieve. Therefore, it can use
internal kernel disk when using removable disk. In chromeos_interface, add a
method to get internal disk for arm or x86.

BUG=chrome-os-partner:13323
TEST=Boot with removable disk, check kernel_handler fetch whick disk device.

Change-Id: Ie8e2f2e9e3b1f72dc8551da37ad26f51a96299b1
Reviewed-on: https://gerrit.chromium.org/gerrit/31692
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Commit-Ready: Chun-Ting Chang <ctchang@chromium.org>
Tested-by: Chun-Ting Chang <ctchang@chromium.org>
2 files changed