… support VS gearbox phy feature

* scripts and configuration needed to support a second syncd docker (physyncd)
* physyncd supports gearbox device and phy SAI APIs and runs multiple instances of syncd, one per phy in the device
* support for VS target (sonic-sairedis vslib has been extended to support a virtual BCM81724 gearbox PHY).

HLD is located at https://github.com/Azure/SONiC/blob/b817a12fd89520d3fd26bbc5897487928e7f6de7/doc/gearbox/gearbox_mgr_design.md

**- Why I did it**

This work is part of the gearbox phy joint effort between Microsoft and Broadcom, and is based
on multi-switch support in sonic-sairedis.

**- How I did it**

Overall feature was implemented across several projects. The collective pull requests (some in late stages of review at this point):

sonic-net/sonic-utilities#931 - CLI (merged)
sonic-net/sonic-swss-common#347 - Minor changes (merged)
sonic-net/sonic-swss#1321 - gearsyncd, config parsers, changes to orchargent to create gearbox phy on supported systems
sonic-net/sonic-sairedis#624 - physyncd, virtual BCM81724 gearbox phy added to vslib

**- How to verify it**

In a vslib build:

root@sonic:/home/admin# show gearbox interfaces status
  PHY Id    Interface        MAC Lanes    MAC Lane Speed        PHY Lanes    PHY Lane Speed    Line Lanes    Line Lane Speed    Oper    Admin
--------  -----------  ---------------  ----------------  ---------------  ----------------  ------------  -----------------  ------  -------
       1   Ethernet48  121,122,123,124               25G  200,201,202,203               25G       204,205                50G    down     down
       1   Ethernet49  125,126,127,128               25G  206,207,208,209               25G       210,211                50G    down     down
       1   Ethernet50      69,70,71,72               25G  212,213,214,215               25G           216               100G    down     down

In addition, docker ps | grep phy should show a physyncd docker running.

  Signed-off-by: syd.logan@broadcom.com

…ific code not relevant

This reverts commit e8958db.

…to upstream-merge

…tings

…kvm_x86_64-r0