DELL S6000-ON: Update INSTALL instructions

This patch updates the INSTALL instructions for the DELL S6000-ON platform.

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Installing ONIE on DELL S6000 Series w/ Intel Atom Centerton (S1220) CPU Module
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Cross-Compiling ONIE
====================

To compile ONIE first change directories to ``build-config`` and then
type ``"make -j12 MACHINEROOT=../machine/dell MACHINE=dell_s6000_s1220 all"``.

  For example::

  $ cd build-config
  $ make -j12 MACHINEROOT=../machine/dell MACHINE=dell_s6000_s1220 all

When complete, the ONIE binaries are located in
``build/images``::

  -rw-r--r-- 10487413 Mar 27 11:01 onie-updater-x86_64-dell_s6000_s1220-r0
  -rw-r--r-- 24620032 Mar 27 11:01 onie-recovery-x86_64-dell_s6000_s1220-r0.efi64.pxe
  -rw-r--r-- 22020096 Mar 27 11:01 onie-recovery-x86_64-dell_s6000_s1220-r0.iso

It builds image for update via ONIE,image for boot via pxe and image for booting via USB.

Complete installation details, refer to machine/dell/dell_s6000_s1220/INSTALL.