[Mellanox] [SONiC BMC] Support BMC side and align host side for usb0 configuration

[benle7]

Why I did it

This change enables SONiC BMC USB link configuration on Mellanox Switch-BMC platforms and aligns
Switch-Host detection logic with the new runtime BMC infrastructure. The goal is to use consistent
role-based checks (switch_host / switch_bmc) and runtime BMC data, instead of legacy platform-file
presence assumptions.

Work item tracking

• Microsoft ADO (number only):

How I did it

• Added BMC-side platform data for AST2700:
  • device/aspeed/arm64-aspeed_nvidia_ast2700_bmc-r0/bmc.json
  • updated device/aspeed/arm64-aspeed_nvidia_ast2700_bmc-r0/platform_env.conf to switch_bmc=1
• Added host-side role markers:
  • device/mellanox/x86_64-nvidia_sn6600_ld-r0/platform_env.conf with switch_host=1
  • device/mellanox/x86_64-nvidia_sn6810_ld-r0/platform_env.conf with switch_host=1
• Updated Mellanox platform API logic to use the new host/runtime detection path:
  • device_data.is_platform_with_bmc() now checks is_switch_host() + get_bmc_data()
  • bmc.py now gates _get_bmc_values() via DeviceDataManager.is_platform_with_bmc()
  • component.py CPLD aux-power path now uses DeviceDataManager.is_platform_with_bmc()
• Aligned unit tests with the new detection path:
  • tests/test_bmc.py
  • tests/test_component.py

How to verify it

• Switch-Host side:
  • verify platform_env.conf has switch_host=1
  • verify /etc/sonic/bmc.json is present after boot
  • verify usb0 comes up with host-side IP from bmc_if_addr
  • verify host <-> BMC ping on the USB link
• Switch-BMC side:
  • verify platform_env.conf has switch_bmc=1
  • verify /etc/sonic/bmc.json is present after boot
  • verify usb0 uses BMC-side IP from bmc_addr
  • verify BMC <-> host ping on the USB link
• Run relevant unit tests:
  • pytest platform/mellanox/mlnx-platform-api/tests/test_bmc.py
  • pytest platform/mellanox/mlnx-platform-api/tests/test_component.py

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).