Changed mirror paths in the layered build guide and layered build
reference to remove hard-coded references to stx-r1, pike, and
cgcs-centos.
Added review comments.
These edits are a follow-on to Build Guide review: 757932
Closes-Bug: 1883760
Change-Id: I898e3bf387fb28a5c2e9f76713cf50fa19ce8fe6
Signed-off-by: MCamp859 <maryx.camp@intel.com>